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The fate of several Tennessee incumbents hangs in the balance in Thursday’s Republican primary, but it may not be Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderThe risk of kicking higher ed reauthorization down the road Maternal deaths keep rising in US, raising scrutiny Supreme Court weighs future of online sales taxes MORE (R) who is sweating the most.

The veteran politician is expected to easily survive a Tea Party challenge from state Rep. Joe Carr (R), who never really caught on with the national conservative base. But two other House members might not be as lucky and could end up in the nail-biters of the night.

Embattled Rep. Scott DesJarlais (R-Tenn.) looked like a goner, under fire amid a past abortion scandal that only came to light two years ago. State Republicans would like to see state Sen. Jim Tracy oust the incumbent and rid them of an unnecessary headache, but in the final stretch the race has been closer in DesJarlais’s favor than many anticipated.

Two-term Rep. Chuck Fleischmann (R-Tenn.) faces a rematch with Weston Wamp, son of the district’s former congressman. The 27-year-old has actually been running to the incumbent’s left and making a plea for Democratic voters, an unusual move in a GOP primary.

Here’s your cheat sheet for what to watch when polls close across the Volunteer State at 8 p.m. ET.

What will Alexander’s margin be?

The biggest question in the Senate GOP primary isn’t whether Alexander wins — it's by how much.

Carr got the backing of some conservative stars like radio host Laura Ingraham and former vice presidential nominee Sarah Palin, but the major national groups never came to his aid. And though Carr has gotten the most Tea Party buzz, there are still five other names on the ballot  all diluting Carr’s vote and boosting Alexander’s chances.

Even Alexander’s detractors in the state acknowledge the former governor is a Tennessee institution.

Still, his voting record has given conservatives an opening to take some of the shine off Alexander. They’ve pointed to his work on the bipartisan immigration reform bill and his not standing with the Tea Party wing in last year’s ObamaCare funding standoff.

Alexander dismissed those critics, telling The Hill last month in an interview that “I’m not in the shut-down-the-government crowd, I’m in the taking-over-the-government crowd.”

Also crucial to his success is the fact that he never lost touch with his home state, as other vulnerable veterans did, and ran a vigorous campaign from beginning to end, finishing this week with a bus tour. He’s also crushed Carr on air and the ground, even though the insurgent did have some help from a questionable super-PAC.

According to Republicans in the state, if Carr has a stronghold anywhere it’s in some of the collar counties around Nashville. He’s from just south of the state capital in Murfreesboro and will likely perform best in Middle Tennessee, a more transient area not filled with Alexander loyalists.

Alexander’s bread and butter has always been in his native East Tennessee, the most Republican part of the state and home to all the statewide elected officials. If he’s racking up big numbers there, it could be an early night for the incumbent.

Carr faces a challenge not from Alexander but from self-funding radiologist George Flinn in West Tennessee. He’s run twice for Congress from the area and owns several radio and TV stations too. Flinn could post pockets of support in Shelby County (Memphis) and split the anti-Alexander vote even more.

Will the DesJarlais scandal stick?

Two years ago, it seemed all but certain that DesJarlais, a Republican elected in the 2010 wave, was on his way out. Just ahead of November 2012, revelations came to light that he had pushed his ex-wife to have two abortions and carried on affairs with patients, one of whom he urged to terminate a pregnancy.

Social conservatives in the district and nationally were outraged, and state Sen. Jim Tracy (R) jumped in the next January and outspent and outraised DesJarlais.

But in the long primary’s closing days, many state observers were beginning to openly fret DesJarlais could somehow win the race he was long supposed to lose.

The congressman was diagnosed with cancer last month, limiting his time on the campaign trail but worrying some Tracy supporters that Tracy’s negative attacks could backfire. DesJarlais apologized for the incidents, reaching out to several faith communities to ask for forgiveness.

Tracy’s final ads reminded voters that DesJarlais was fined by the state medical board for relationships with patients and says he “no longer has credibility.”

But Tracy’s final push seems to be assuaging some of those who once worried the race was moving away from him. Several sources on the ground who were once thought DesJarlais was in the driver’s seat now point to a significant momentum shift in the final days and aren’t as concerned as they once were.

Early voting turnout in Tracy’s state Senate district, which takes in nearly half the congressional district, has been especially strong. If he’s racking up big percentages in Rutherford County (Murfreesboro) and his native Bedford County, it’s a good sign.

DesJarlais has been hampered on the campaign trail by his cancer treatments but has been holding tele-town halls, with one on Tuesday drawing nearly 12,000 participants. The South Pittsburg physician has always had a more rural base, and he needs those voters to turn out on Election Day. Typically early voting is less strong in more rural areas, so look to Marion and Franklin counties especially. If DesJarlais isn’t doing well in those counties along the Georgia/Alabama border, it’s over.

Can Weston Wamp regain his father’s seat?

Since winning a multi-candidate primary four years ago to replace Rep. Zach Wamp (R), who was making an unsuccessful bid for governor, Chuck Fleischmann’s grip on the Chattanooga-based 3rd District has never seemed that strong.

The incumbent only took 30 percent in an 11-way primary in 2010, and last year netted just 39 percent in the four-way race.  

Part of that margin in 2012 was thanks to Wamp’s son, Weston, who is trying for a second time to unseat Fleischmann and prove the Wamp name is still powerful in East Tennessee.

Trying to recover from a weak debate performance, Fleischmann has seized on statements Wamp made suggesting he would support amnesty for illegal immigrants and pointed out he’s even stumping for cross-over Democratic votes in the open primary. He unleashed a barrage of negative attacks, but his final one hit Wamp’s business incubator for investing in a health insurance firm that profited from ObamaCare.

The nasty tone of the race seemed to underscore just how tight this one-on-one match was becoming. State Republicans say they think the race has moved slightly in Fleischman’s favor in the closing days as well, but some say this could be the closer one to watch on Election Night.

In 2012, Wamp narrowly won Hamilton County, the district’s most populous. He needs to win there as well and cut into Fleischmann’s expected advantage in the more rural northern counties. Flesichmann is also expected to do well in McMinn and Anderson counties, which he lost last time to dairy magnate Scottie Mayfield. Mayfield is backing Fleischmann now though and is appearing in a TV ad alleging Wamp secretly recorded a conversation, raising questions about his character.

—This post was updated at 11:17 a.m.