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

Today, in Tennessee, the tea party has its last-best chance to beat an incumbent senator in a race that's a marquee match-up with the GOP establishment. After millions of dollars, the tea party failed to topple incumbent senators in Kentucky, South Carolina, Mississippi, and most recently earlier this week in Kansas. If Tennessee Sen. Lamar Alexander beats back his tea party challenger state Rep. Joe Carr, this will be the first cycle since 2008 when no incumbent GOP Senator has lost a re-nomination fight.

WHO'S ON THE BALLOT?

The Senate battle is not the only fight in Tennessee today, with an embattled congressman trying to hold on and a former congressman's son trying to get his father's seat back, and much more. Our friends at FiveThirtyEight.com have joined us again to explain the importance of today's big race. Look for FiveThirtyEight.com senior political writer Harry Enten's take below.

TEA PARTY'S LAST STAND: Sen. Lamar Alexander's 35 years in office include two terms as governor beginning in 1979 and two terms in the Senate with two presidential runs mixed between. He also served as the president of the University of Tennessee from 1988 to 1991, but he has two primary challengers today. WHY IT MATTERS? Alexander's main foe in the Tennessee Senate GOP primary is state Rep. Joe Carr, while Memphis radio station owner and self-funded candidate George Flinn is a minor candidate who actually considers himself "friends" with the 74-year-old incumbent. Of course, Flinn could still be a spoiler, likely hurting Carr. Carr, 56, has tried to paint Alexander as out of touch with the conservative state, but that argument has already failed in the other states where tea party challengers tried to beat incumbents. Alexander also nabbed endorsements early and has stressed his ability to get things done in Washington by working across the aisle, including on the polarizing issue of immigration reform. Carr is backed by Sarah Palin and radio talk show host Laura Ingraham, who helped Dave Brat beat Rep. Eric Cantor in the shocker of the cycle. The financial disparity between Alexander and Carr is quite wide with Alexander raising over $6.6 million to Carr's $1.1 million. Carr has been going after Alexander on immigration, criticizing him for his vote last year for the Senate's comprehensive immigration legislation that includes a pathway to citizenship for undocumented immigrants already in the country. Carr has accused Alexander of supporting amnesty, especially over the last weeks as the crisis on the border grabbed headlines. There has been scarce polling in the race, but Alexander looks to be safe. Of course, even this week Sen. Pat Roberts' victory was much tighter than expected.

FiveThirtyEight's Take: Polling Past polling in Tennessee primaries has not been terribly accurate. Errors of five points or greater occurred regularly in the 2006 senatorial primary, 2008 presidential primary, 2010 gubernatorial primary, and 2012 presidential primary. The chance for a surprising result, either a blowout or Carr winning, should not be discounted. Key Counties: Alexander should be strongest in the eastern part of the state, as is the pattern with establishment Republicans. Alexander performed best there in his competitive 2002 senatorial primary against Ed Bryant. The east is anchored in-and-around Knoxville (Knox County) and also includes Hamilton County (Chattanooga) and the Tri-Cities region (Sullivan and Washington counties) in the northeast. Carr will be looking to run up the score in the central part of the state. He represents part of Rutherford County, which is to the southeast of Nashville. It was one of Rick Santorum's best counties in the 2012 presidential primary. Carr should also do well in next door Wilson County, which was among Romney's worst counties and is home to over 2 percent of the primary vote in 2012. Swing County: Williamson, next to Nashville, in the center of the state could be key. Williamson is home to about 5 percent of primary voters. Alexander lives in Nashville, and Romney's best counties were Davidson and Williamson. However, Alexander lost Williamson in 2002, and Carr's home district is no more than a 40-minute car ride from Nashville.

EMBATTLED CONGRESSMAN TRIES TO SURVIVE: Tea-partier and embattled Rep. Scott DesJarlais is an incumbent with one of the steepest uphill climbs this primary season. A physician, he was re-elected in 2012 despite jaw-dropping revelations that he dated patients and urged one to have an abortion. He has campaigned and governed on an anti-abortion rights platform. After the election in 2012, it was also revealed DesJarlais was in favor of his ex-wife's choice to have two abortions and he had eight affairs, as well as revelations that he encouraged a girlfriend to get an abortion and used a gun to intimidate his first wife during a fight. Last year he was fined and reprimanded by the Tennessee Board of Medical Examiner for having sex with his patients. His challenger in the GOP primary for Tennessee's fourth district, state Senator Jim Tracy, is hoping that voters in this district will remember these scandals when they go to the polls tonight. Tracy has questioned whether DesJarlais can be an effective Congressman with these kind of moral lapses. He has also outspent DesJarlais about four-to-one in the weeks leading up to this primary. DesJarlais has dismissed the revelations about his personal life as "old news," but he has failed to get the kind of establishment support incumbents usually attract. Don't count him out, though. With little to no available polling and campaigning on his conservative record in this red state, there's no way to know how this will go.

THE OTHERS:

DEMOCRATIC PRIMARY FOR TENNESSEE'S NINTH DISTRICT: Incumbent Democratic Rep. Steve Cohen represents this predominantly African-American district, which includes Memphis. Ricky Wilkins, an African American attorney who currently serves as chairman of the Memphis Housing Authority, is challenging the white Jewish Congressman on the premise that he can better represent and relate to the district's population. Cohen was first elected in 2006 and Wilkins is Cohen's most aggressive Democratic primary challenger in years. He has won every primary election by more than 70 percent since he was first elected and is widely favored to win, but with little available polling we'll be watching for an upset. The president has backed the four-term incumbent and Memphis native. Cohen grabbed national headlines last year after a Twitter exchange during the president's State of the Union address with a woman he identified as a daughter he only found out about three years earlier. A paternity test months later revealed he was actually not the father of the aspiring swimsuit model.

REPUBLICAN PRIMARY FOR TENNESSEE'S THIRD DISTRICT: Rep. Chuck Fleischmann is another incumbent facing a Republican primary challenge, but this one is from the 27-year-old son of the district's former congressman: Weston Wamp, the son of Fleischmann's predecessor Zach Wamp who represented the district from 1995 to 2011. The younger Wamp is not a tea-party challenger and is instead running to the center pledging to work across the aisle with his campaign website saying Wamp "doesn't believe party fanaticism is the right path forward for the country." Fleischmann, 51, may have the edge of incumbency and has outraised his opponent, but Wamp has received notable endorsements from Rick Santorum and Retiring GOP Sen. Tom Coburn from Oklahoma. National Republican Senatorial Committee Chairman Sen. Jerry Moran also donated to Wamp, which is surprising as the NRSC has been defending incumbents from primary challenges all season. The NRA and the National Right to Life Coalition have both backed Fleischmann who has attacked Wamp for not being critical of President Obama and accused Wamp of supporting "amnesty" for undocumented immigrants. This election is a rematch of their 2012 bout when Fleischmann won 39 percent of the vote to Wamp's 29 percent, placing third in the four-way contest.

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