This contest is the most expensive Senate race so far this cycle in terms of money raised -- with both sides raising more than $50 million total.
Spending is very high, as well. More than $33 million has been spent, making it the second-most-expensive Senate race in the country.
The figures are underscored by the fact that both candidates made a pact in January agreeing not to accept advertising by outside groups.
Warren prides herself on her character. She is a Harvard professor with an up-by-her-bootstraps story and a father who worked as a janitor in Oklahoma. But her background is what sparked "Fauxcahontas" -- a controversy over Warren's past assertions of Native American heritage and whether she used that minority status for professional advancement.
It was something of a political miracle when Scott Brown won the right to complete Ted Kennedy's Senate term two years ago, but now the incumbent senator is finding out how hard it is for a Republican to win again in Massachusetts.
Brown is a moderate who often defies his own party. But as Warren reminds voters often, a vote for Brown is also a vote possibly to put Republicans in charge of the Senate.
Brown faces two large challenges: the heavily Democratic makeup of the state's electorate (48 percent Democrats to 12 percent Republicans) and the wide margin by which Obama is leading in the state, though it is Mitt Romney's home state.
Brown has to win a huge percentage of independent voters, who make up about 40 percent of the state's voting population.
The race is a toss-up.
Two former governors are facing off in a race that is going to be defined more by the top of the ticket than anything else, in a seat left open when Sen. Jim Webb, D-Va., decided to retire in 2012.
Former Virginia Republican Sen. George Allen trying to reclaim his seat he held from 2001 to 2007. He is running against Tim Kaine, the former Democratic National Committee chairman and also a former Virginia governor.
Both candidates have decent favorability scores, and both are very well funded.
This Senate race, some say, is a proxy for presidential race. There are not likely to be many Obama-Allen voters or Romney-Kaine voters.
Virginia is poised to be hit very hard by the looming threat of sequestration in Congress, the year-end intersection of the expiration of the Bush-era tax cuts, the $1.2 trillion in spending cuts Congress imposed on itself when it failed to come up with a deficit reduction plan, and expiring payroll tax breaks.
Nearly 200,000 jobs could be lost in Virginia if Congress does not find a way around the sequester, as many jobs in the state depend on military spending.
The race has been all but tied for the entire election season and will be very close.
The race is a toss-up.
The winner of this state in the race for the presidency might also lift the party's Senate nominee to victory.
Republican candidate Josh Mandel is an Iraq War veteran who served two tours in Iraq. At 34, he serves as the Ohio state treasurer, and he has been a Cleveland-area city councilman and state legislator.
The young challenger is running against the incumbent Democratic senator, Sherrod Brown.
Democrats have tried in the race to cast Mandel as an absentee state treasurer while he's running for higher office. Mandel has said that new blood needs to be in representing Ohio in Washington.
Brown's record has made him a target for Republicans. In 2011 National Journal ranked Brown the fifth-most-liberal senator, tied with four other senators.
Mandel has been a great fundraiser for the Republicans, but Republican groups have also poured money into the state, spending as much as $20 million to get Mandel elected.
Meanwhile, Democrats have also poured money into the state, spending about $8 million for Brown to hold on to his seat.
Sen. Jon Tester, who leans to the right edge of the Democratic Party, is up against the state's only congressman, Rep. Danny Rehberg, R-Mont. Both candidates have deep roots in Montana, and they are battling it out to define the other as more ingrained in Washington culture in this reliably red state.
The incumbent senator has tried to distance himself from President Obama over the course of the campaign, highlighting times that he "took on the Obama administration" with votes in Congress siding against the administration.
But Rehberg has tried hard to connect Tester to Washington and the White House, highlighting key votes with the administration on marquee pieces of legislation such as the Health Care Reform Act and the stimulus.
Outside groups have poured more than $12 million in attack ads into this race.
This seat became available after longtime Sen. Olympia Snowe announced her retirement in February 2012, leaving a scramble to be her successor.
In a state where 40 percent of voters aren't members of either party, the independent candidate in this race, Angus King, is making headlines.
King, Maine's governor from 1995 to 2003, has yet to divulge the party he would caucus with if elected into the Senate.
"I've let it be known that I really don't want to have those conversations," King has said when asked.
Republican Charlie Summers, 52, is a Navy reservist who served in Iraq and Afghanistan and comes from a small business background.
The contest has split the Democratic national and state parties, where local Maine Democrats are backing their state senator, Cynthia Dill. But National Democrats have held off endorsing her.
Recent polling has shown a tightening race.
Incumbent Republican Sen. Dean Heller, who replaced embattled Sen. John Ensign, is challenged by Rep. Shelley Berkley.
Berkley has been the subject of a House Ethics Committee investigation into whether she used her office to push policies that benefited her husband's medical practice, which her opponent has brought up often on the campaign trail.
Heller supported Republican vice presidential nominee Paul Ryan's Medicare proposals, which Berkley uses against him on the campaign trail.
Republican Heller has attempted to distance himself from the Republican presidential nominee, Mitt Romney, at times. Most recently, he disavowed Romney's famous "47 percent" remarks.
"My mom was a school cafeteria cook, so I have a very different view of the world," Heller said in the U.S. Capitol regarding the controversy over Romney. "I do believe the federal government has certain responsibilities. One of those responsibilities is building bridges and roads, and national defense, but I also believe in a safety net for individuals who need the help, so that's why I would respectfully disagree with the comments that he made."
When longtime Democratic Sen. Herb Kohl announced in May 2011 he would not be running for reelection, it left his seat, which he has occupied since 1989, up for grabs.
Former four-term Republican Gov. Tommy Thompson is up against Democratic Rep. Tammy Baldwin.
If elected, Baldwin will be the first openly gay senator in U.S. history.
A tremendous amount of money from both inside and outside the state has flowed into the race, with Baldwin out raising Thompson nearly three times over -- $7.1 million to his $2.5 million in the summer.
Individual contributions have accounted for 88 percent of Baldwin's haul.
When longtime Democratic Sen. Kent Conrad announced that he would not seek another term in office, it made his seat available.
This is a fun race that features a very personable Democrat, the state attorney general, Heidi Heitkamp, who overcame breast cancer and is running against the new Republican congressman from the class of 2010, Rep. Rick Berg.
North Dakota has the lowest unemployment rate in the country. Key issues in the state are President Obama's health care bill and the Keystone oil pipeline.
This is a Republican state that Romney will easily win, and that should help bring enough votes to help Berg get elected. But the race will be close, especially because this ruby-red state likes to send Democrats to the Senate.
But Republicans see the state as a good chance to pick up a seat from Democrats.
Currently, ABC News rates this race as a toss-up.
Incumbent Democratic Sen. Claire McCaskill is trying to defend to hold on to her seat against Rep. Todd Akin.
With McCaskill behind in the polls, the race seemed to be going Akin's way until August, when the congressman was trying to explain his pro-life position and set off a firestorm.
During a TV interview, Akin was speaking about the odds of a woman becoming pregnant from rape and said, "If it's a legitimate rape, the female body has ways to try to shut that whole thing down."
The comment was met with immediate criticism, with women's groups and unions lined up in opposition to Akin. Numerous Republicans chastised his comments. Akin would not withdraw from the race despite numerous calls for him to do so.
He recently said that he has no regrets about staying in the race.
"This thing has been a bucking bronco ride," Akin said in an interview with The Associated Press. "It's always worth it when you do what you think in your heart is the right thing to do."
Despite the National Republican Senatorial Committee pulling out money from the state, the Senate Conservatives Fund, a political action committee associated with Sen. Jim DeMint, R-S.C., pledged $290,000 in support of him.
Before Akin's comments, Sen. McCaskill had her share of problems, as well. But McCaskill is up in the polls for the first time in awhile.
Tea party-backed State Treasurer Richard Mourdock defeated Sen. Richard Lugar in a primary upset this year and now faces Democratic Rep. Joe Donnelly, a conservative Democrat.
Mourdock beat Lugar by claiming the longtime senator had become too moderate while in Congress. He has tried to highlight Rep. Donnelly's votes, especially for President Obama's health care bill.
Outside interest groups have poured nearly $8.4 million into this race.
A loss by Mourdock of this historically Republican seat could end the GOP's chances at flipping the Senate -- and his recent controversial comments during a debate could hurt him.
During the debate, Mourdock said pregnancies resulting from rape are part of God's plan, tearfully explaining that he only supports abortions when a mother's life is in danger.
Mourdock clarified his comments in a press release and at a news conference the next day, but he didn't apologize, instead accusing critics of twisting his words.
"God creates life, and that was my point," he said. "God does not want rape, and by no means was I suggesting that he does. Rape is a horrible thing, and for anyone to twist my words otherwise is absurd and sick."
At a news conference later he said that his words were "mistook and twisted," and that the uproar is symptomatic of "what's wrong with Washington."
Incumbent Democratic Sen. Bill Nelson faces challenger Rep. Connie Mack, the son of former Republican Sen. Connie Mack, who served Florida in the U.S. Senate from 1989 to 2001 until he declined to run and was succeeded by Nelson.
Nelson's popularity ratings are weak in the state, but Democrats have painted Mack, a four-term member of Congress from Cape Coral, Fla., who is running his first statewide campaign, as hard partying and not ready for the Senate.
In a report covering Sept. 9-30, $4.5 million was spent on TV spots in the Mack-Nelson race. Outside groups financed 48 percent of the ads, the fourth-highest percentage in 15 Senate races reviewed nationwide.
It's been almost two decades since there was a Democrat in the Senate from Arizona, but that could change.
Dr. Richard Carmona, who was U.S. surgeon general under President George W. Bush, was recruited by President Obama to run for the Senate. He is a decorated Vietnam War veteran and Hispanic.
His rival is Republican Rep. Jeff Flake, a fiscal conservative who has earned a reputation for bucking his party. He has called Carmona, "Barack Obama's rubber stamp."
Arizona remains a solid Republican state. The race leans Republican.
This race has been characterized as one of the most negative, with both candidates bashing the other at every opportunity.
Linda McMahon, the former CEO of World Wrestling Entertainment, has made some headway with her negative ads over Rep. Chris Murphy's missed mortgage and rent payments, and an alleged sweetheart mortgage deal when he was on the House Finance Committee.
Although Murphy has said he want to focus on policy issues, he got a jab in when he pointed out that McMahon was late on her property taxes on her Greenwich home.
In the most recent debate, Murphy accused McMahon of lifting her jobs plan mostly from the Cato Institute and the House Republican websites, but she hit back, saying she had extensively sourced her material.
"Shame on you! You have just accused me of plagiarizing my plan. It is beneath you. You thought this was going to be a coronation, but you're in a serious race with a serious woman."
After losing two years ago to Richard Blumenthal in a bid for the state's other Senate seat, McMahon hopes for a better outcome this time around.
This seat is available after the retirement of longtime Democratic Sen. Daniel Akaka.
Republican Linda Lingle has been casting herself as a moderate in the campaign, hoping to separate voters for Hawaii-born President Obama from their votes for the Senate. Rep. Mazie Hirono, in Congress since 2007, is trying to underscore Lingle's Republican roots and to tie her as much as possible to Republican presidential nominee Mitt Romney.
After Sen. Akaka's retirement, Republicans targeted this race as a potential pick-up.
This seat is up for grabs after the retirement of Republican Sen. Kay Bailey Hutchison.
Former state solicitor general and Tea Party firebrand Ted Cruz is up against former state Rep. Paul Sadler, a Democrat.
Sadler has tried to label his opponent as a hard-right extremist. The pair bickered during their debates, with Sadler saying Cruz's positions were crazy and even calling him a "troll."
Republicans are hoping to hold on to this Republican seat.
This seat is available after the retirement of Democratic Sen. Jeff Bingaman, who announced his decision not to seek another term last year.
This race has been competitive from the very start, matching Democratic Rep. Martin Heinrich against Republican former Rep. Heather Wilson.
Wilson represented New Mexico's 1st congressional district from 1998 to 2009, when she left to run unsuccessfully against Democrat Tom Udall for the Senate. Heinrich succeeded Wilson in the House of Representatives, elected in 2009.
With Wilson and GOP presidential nominee Mitt Romney both trailing in the state, the Republican Party nationally has been shifting resources away from New Mexico to more competitive states.
This race matches up Democratic incumbent Sen. Bob Casey with Republican CEO Tom Smith in a race that has tightened significantly since the end of September.
Sen. Casey is the son of the late former Gov. Robert P. Casey and a former state treasurer and auditor general.
Tea Party favorite Smith is pretty new to the political scene. The only office he's ever held was as supervisor in a township. He made his fortune in coal mining and has run his family farm.
The race has become remarkably close and competitive this fall. Casey has tried to paint Smith as one who would add to the partisanship that marks Congress, and Smith has said that his background has prepared him better to help solve the country's problems.
Avery Miller contributed to this report.