(Daniel Lin/The Enterprise-Journal/AP Photo)

ABC News' Shushannah Walshe and FiveThirtyEight.com's Harry Enten report, with ABC News' Caleb Jackson and Elizabeth McLaughlin:

The primary season continues, and today is one of the biggest days left on the calendar. Voters in eight states are going to the polls: It's primary day in New York, Maryland, Oklahoma, Colorado and Utah. There is also a special election in Florida, and run-offs in both Mississippi and South Carolina. For the first time, our friends at FiveThirtyEight.com have joined us to explain the importance of today's big races. Look for FiveThirtyEight.com senior political writer Harry Enten's take below.

Here are 10 races to watch this primary day:

1. RE-MATCH IN THE DIRTY SOUTH

It's a re-match. Incumbent Sen. Thad Cochran is in the fight of his life against tea party challenger state Sen. Chris McDaniel in the Mississippi GOP Senate primary. This has been called the nastiest primary this year and is the marquee race today. The two first faced off on June 3, but since neither crossed the 50 percent threshold, they will meet again today. The first time around was a nail biter of a race with McDaniel on top with 49.5 percent of the vote to Cochran's 49 percent. Less than 1,400 votes separated the two.

WHY IT MATTERS: Cochran, known as "Gentleman Thad," was elected to the House in 1972, serving three terms before being elected to the Senate, and he could very well fall to the tea party today. Run-offs have been traditionally hard on incumbents. It has been a dirty, and at times bizarre, race. Conservative notables, including Sarah Palin, Rick Santorum, and even longtime game show host Chuck Woolery, have been stumping for the former talk radio show host, all joining his call that Cochran is out of touch, embodies Washington, D.C., and that his time is over. John McCain made a last-minute visit for Cochran and Mississippi native Bret Favre starred in a Chamber of Commerce-funded ad where he said "Thad Cochran always delivers, just like he did during Katrina." Cochran's message is he's the choice that can bring in those federal dollars and disaster relief money, something McDaniel's supporters call nothing more than pork. Joseph Parker, a retired professor of political science at the University of Southern Mississippi, describes McDaniel's mindset as similar to Ted Cruz, a comparison McDaniel often makes on the stump: "I'm going to Washington to fight Washington, not to bring home anything," Parker said. Over $8 million in third-party money has poured into the state from both sides, making this one of the most expensive primaries this cycle. In an interview after the primary former Mississippi Gov. Haley Barbour - who supports a pro-Cochran super pac Mississippi Conservatives - said there would be no backing down in these extra weeks. "We are not going to let a bunch of people from Washington or New York dictate who represents Mississippi in the U.S. Senate," Barbour said. If Cochran does lose, Democrats will immediately try to tout their candidate, former Rep. Travis Childers. It may seem impossible in deep red Mississippi, but Barbour's super pac has already said McDaniel would be "embarrassing" for the state. Expect Democrats to repeat those calls.

FiveThirtyEight's Take: In the first round, Cochran ran up the score in four types of counties. He did best in his home county of Lafayette in the north; the counties with large African-American populations along the Mississippi Delta in the northwest; Hinds and Madison counties around the highly populated city of Jackson in the center of the state; Harrison County on the Gulf of Mexico and Lauderdale County in the east, which are anchored by military bases. McDaniel did best in two places. His home county of Jones in the southeast provided him a nearly 10,000 vote majority - by far the largest margin in any county. Anything surrounding Jones County is McDaniel country, and voters turned out in large numbers in the first round. He also scored big in the swing county of DeSoto in the northwest. Swing County: Rankin is right near Jackson, but African-Americans make up a much smaller percentage of the population than in Hinds or Madison. Cochran won the county by two points in the first round. Is Cochran's Strategy of Courting Black voters working? Claiborne and Jefferson counties in the southwest should tell the story. African-Americans make up about 85 percent of the population in both counties. Cochran won only 161 and 121 votes respectively in Claiborne and Jefferson in the first round. If he has expanded the electorate, Cochran will be doubling if not tripling those totals in the runoff.

2. JUDGMENT DAY IN HARLEM

It's judgment day - again - for 84-year-old Charlie Rangel in the Democratic primary for New York's 13th District. He's meeting his 2012 opponent, Dominican-American State Sen. Adriano Espaillat one more time, as well as Rev. Michael Walrond Jr. The 13th District located in Harlem has always been predominantly African American, but is now majority Hispanic, a slight demographic shift that could give Espaillat the advantage this go-around. He would also be the first Dominican-born member of Congress. In the 2012 Democratic primary, Rangel won by only around 1,000 votes, and this is the toughest fight of his political career.

WHY IT MATTERS? Rangel, who was first elected to represent the district in 1970, defeated the legendary Adam Clayton Powell Jr. by a tiny margin after some ethics questions in Powell's political career. Trying to return to Congress for a 23 rd term, Rangel too has been dogged by ethics questions and was forced to resign as the first African-American chairman of the House Ways and Means Committee in 2010 after a series of ethical lapses involving improper fundraising, failure to pay taxes, among other low points in his career. Rangel, who was arguably one of the most powerful Democrats in Congress at one point, has not regained his former political rank, but the ethics allegations are not new and dogged him in 2012 as well. Rangel was censured by his own colleagues and he was still able to beat Espaillat. This primary season has been a hard one on elder statesmen (see: Thad Cochran above, as well as Ralph Hall in Texas) and the New York Times endorsed Espaillat with a call for Rangel to "yield to the next generation." New York Gov. Andrew Cuomo came in with a last-minute endorsement for Rangel, but he can stop waiting for President Obama's. That won't happen. Democratic National Committee press secretary Michael Czin said in a statement, "Like 2010 and 2012, the President will not be endorsing in this race. However, he believes that Mr. Rangel has been and continues to be an advocate for quality, affordable health care, fair wages and opportunity for all his constituents." Rev. Michael A. Walrond Jr., the long shot in this race, serves as the pastor of First Corinthian Baptist Church in Harlem, but he has failed to garner much support outside of his congregation.

FiveThirtyEight's Take: Rangel, who is half African American, is banking on a large turnout from African Americans in the heart of Harlem. Polling puts him ahead by 70 points among this group. Rangel, who is also half-Puerto Rican, is hoping to repeat his strong 2012 performance among older Puerto Ricans in Spanish Harlem in the southeastern part of the district. Espaillat, who is Dominican, needs strong turnout from the Dominican-heavy northwestern part of the district that he represents in the state Senate. In 2012, Espaillat's strength here nearly was enough to upset Rangel. Espaillat needs to improve in the Latino-heavy Bronx, in which he only won a four-point plurality in 2012 and the county organization is behind him. Swing Group: White voters in the far western portion of the district along the Hudson River will be key. They'll make up about the same 25 percent of voters that African Americans will, but less than the 40 percent Hispanics will.

3. SOONER STARS

The battle for Sen. Tom Coburn's seat looks like a familiar establishment vs. tea party brawl with 46-year-old Rep. James Lankford fighting 36-year-old Rep. T.W. Shannon, the former speaker of the Oklahoma House and a fast rising star in the GOP. But, the lines don't fall in such straight lines with this race.

WHY IT MATTERS? Lankford won his seat in the 2010 tea party wave and has since ascended to the House Republican leadership. Shannon has the backing of both Ted Cruz and Sarah Palin, who stumped on his behalf calling him, "the whole package." He was the first African-American (he is also part native American) and youngest Speaker of the Oklahoma House and there is no doubt no matter what happens today he will have a long career in the party, with allies in both the establishment and tea party. Lankford and Shannon top a list of seven candidates seeking the GOP nomination, which includes state Sen. Brogdon and four other candidates. If neither Lankford or Shannon clear the 50% threshold today, they will head to an Aug. 26 run-off. Oklahoma's senior senator, Republican Sen. Jim Inhofe, is seeking a fourth term and he too faces a challenge in the GOP primary, four of them. This is seen in the state as the "other" Senate race and Inhofe is widely favored to win. The winner in both races today is the likely the November winner in this deep red state.

FiveThirtyEight's Take: Polling and common sense tell us that Lankford will do best in the highly populous Oklahoma County and surrounding counties, which is the area he represents in Congress. Oklahoma County has also been slightly more favorable to the more "moderate" candidates in past presidential primaries, which should be benefit Lankford as well. Polling suggests that Shannon will do best in the eastern part of the state. Santorum did very well in the east in the 2012 Republican presidential primary. This suggests the more conservative candidate does better in this area. The east also happens to be home to a large percentage of American Indians, though many are not members of Shannon's Chickasaw Nation. Additionally, Shannon should thrive in his home county of Comanche in Southwest Oklahoma. Swing Areas: The second-most populous county in the state is Tulsa, and neither candidate can claim it as home. Polling from the area has been mixed on who is ahead. If Lankford wins here, he'll probably win the most votes statewide. The third most-populous county of Cleveland is just to the south of Oklahoma County. It's not in Lankford's congressional district, and it has leaned somewhat more to the conservative side than Oklahoma County in Republican presidential primaries.

4. MARYLAND'S CHOICE

With current Maryland Gov. Martin O'Malley not running for re-election due to term-limits, the field is wide open for candidates seeking his job. Seven candidates: three Democrats and four Republicans are looking to land a spot on November's ballot.

WHY IT MATTERS? The three candidates in the Democratic primary are Maryland's Lieutenant Gov. Anthony Brown, Maryland's Attorney General Doug Gansler, and State Delegate Heather Mizeur. The four Republican candidates are Harford County Executive David Craig, State Delegate Ron George, and businessmen Larry Hogan and Charles Lollar. O'Malley was on a different type of campaign trail over the weekend, introducing himself to Iowans as he weighs a 2016 presidential bid, but he was back in Maryland in the hours before the primary stumping for his choice, his lieutenant governor Anthony Brown. Brown would be the state's first black governor and the first lieutenant governor to ascend to the top spot. Gansler's blunt talk has gotten him in trouble during the campaign and he grabbed national headlines last year when a photo of him was taken of him at a raucous party full of teenagers. He said he was there to talk to his son and it wasn't his responsibility to break the party up. He is also known for prosecuting the Washington, D.C., snipers when he was Montgomery County state's attorney. As for Mizeur, if elected she would be the state's first woman and first openly gay governor. She is considered the most progressive candidate and has called for legalization of marijuana, mandated paid sick leave, and higher taxes on the wealthiest residents.

FiveThirtyEight's Take: Consider this the race that never was. Lt. Gov. Brown was supposed to face a tough race from state Attorney General Gansler. Brown, though, has never led by less than 14 points and was ahead 46 percent to 23 percent in the latest Washington Post poll. Mizeur, a member of the Maryland House of Delegates, was even further back at 16 percent. Brown is a heavy favorite in the fall over whomever Republicans nominate.

5. WILL IT BE TANCREDO?

Four Republican candidates are looking to win today's primary election in Colorado for a chance to take on Democratic incumbent John Hickenlooper in November .

WHY IT MATTERS? Former congressman Tom Tancredo grabbed most of the headlines ahead of today's primary. Known for his inflammatory stance against illegal immigration and recently of his embrace of marijuana legalization as well as his calls for President Obama to be impeached, the Republican Party in the state is hoping for less of a firebrand choice. Some have even called on Tancredo to get out of the race. He's facing off against another former congressman, Bob Beauprez, as well as Colorado's Secretary of State Scott Gessler, and former Colorado Senate Minority Leader Mike Kopp. Beauprez is seen as the Republican establishment candidate, while the others have all chosen to align themselves with different tea party factions.

THE OTHERS:

6. NEW YORK'S 21 ST CONGRESSIONAL DISTRICT GOP PRIMARY

Voters in upstate New York will decide today between former President George W. Bush and Rep. Paul Ryan aide Elise Stefanik and businessman Matt Doheny in the race to replace the retiring Democratic Rep. Bill Owens.

WHY IT MATTERS? The two have similar policy view points, but Doheny has tried to portray Stefanik as a Washington insider who just moved to the district to run, while Stefanik points out Doheny has also worked outside of the district, spending time on Wall Street. Stefanik has the backing of Ryan, she was an aide to his vice presidential campaign helping with debate prep and he came to the district to campaign for her. The 29-year-old also has the backing of Mitt Romney. And in an unusual move, the American Crossroads super pac founded by Karl Rove funded attack ads against Doheny, calling him "unfit for Congress." This is the first time the group has intervened in a GOP House primary. The victor will face Democrat and documentary filmmaker Aaron Woolf in November. No matter what happens today, both Stefanik and Doheny will be on the ballot as Stefanik already has the Conservative Party line and Doheny has the Independence Party line.

7. NEW YORK'S 22 nd CONGRESSIONAL DISTRICT GOP PRIMARY

Incumbent Rep. Richard Hanna is trying to stave off a challenge from state Assemblywoman Claudia Tenney, who the National Review called "The Next Dave Brat.

WHY IT MATTERS? The two-term incumbent is battling Tenney, who has tea party support, but he has the support of a Republican super pac that has spent over $500,000 in ads against Tenney. Supporters of Tenney have tried to portray Hanna as out of touch and not in the district enough, repeating some of the same allegations thrown at Eric Cantor. There is no Democrat on the November ballot, but if Hanna loses, his name will still appear on the ballot in November on the Independence Party ballot line. If Tenney loses, her name will not appear.

8. COLORADO'S FOURTH DISTRICT GOP PRIMARY

Four candidates are seeking to replace Rep. Cory Gardner, who is challenging Sen. Mark Udall. Weld County District Attorney Ken Buck, who had been running against Udall, dropped out when Gardner made his announcement and Buck decided to run for his seat

WHY IT MATTERS? Buck, the unsuccessful GOP challenger to Michael Bennet in the 2010 Senate race in the state, leads in fundraising. He's facing off against state Sen. Scott Renfroe, businessman and former mayor of Cranston, Rhode Island, Steve Laffey, and Weld County Commissioner Barbara Kirkmeyer. Buck is seen as the frontrunner because of his fundraising and previous run, but he lost in 2010 because he was seen as too extreme on social issues. Not so in this primary where Renfroe has called Buck not conservative enough on anti-abortion rights, something Buck denies. Registered Republicans in the district far outnumber registered Democrats in the district making the winner here today in a good position.

9. COLORADO'S FIFTH DISTRICT GOP PRIMARY

Four-term incumbent Rep. Doug Lamborn is facing a primary challenge from retired Air Force General Bentley Rayburn. And after Eric Cantor's stunning loss earlier this month, all incumbents are looking over their shoulder.

WHY IT MATTERS? This is Rayburn's third time running against Lamborn after failed attempts in both 2006 and 2008, but he is hoping to channel some of the same anti-incumbent energy in this Colorado Springs district that helped Brat defeat Cantor in Virginia.

WHAT ELSE ARE WE WATCHING?

10. SPECIAL ELECTION FOR FLORIDA'S 19 TH CONGRESSIONAL DISTRICT

Voters in this southwest Florida district will pick a replacement for GOP Rep. Trey Radel, who resigned in January after pleading guilty to cocaine possession. Republican Curt Clawson, a former CEO of an aluminum wheel company, is the favorite. He is facing off against Democrat April Freeman, who does product placement for television and movies, and Libertarian and former Air Force Captain Ray Netherwood. Republicans have a heavy voter registration in the state and last elected a Democrat to Congress in 1970. Whoever wins will serve out the last six months of Radel's term and then run for re-election in November for the full term.

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