Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeRepublican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style Bush endorsing Biden? Don't hold your breath MORE’s (R-Ariz.) abrupt retirement announcement last week means that all eyes are now on Nevada’s Senate primary, the next key battleground between the GOP establishment and party insurgents.

Danny Tarkanian, who is challenging Sen. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (R-Nev.) in the primary, represents the best chance after Flake’s retirement for grass-roots voters unhappy with Senate Republicans to take down another incumbent.

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While Flake had been considered the Republican most vulnerable to an intraparty challenge, his decision to forgo reelection means Heller is now the most vulnerable GOP senator still on the ballot.

Heller, ranked the 26th most conservative Senator by Heritage Action for America's scoring from the last Congress, might not normally be on the top of the chopping block. But his high profile snubbing of President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE during the 2016 campaign has drawn the ire of Trump supporters, threatening his chances in next year’s primary.

Tarkanian is expected to win the backing of former White House chief strategist Stephen Bannon and his allies who are eager to take incumbent Republican scalps. But Tarkanian’s ties to Bannon are drawing him into the burgeoning national war between Bannon and the GOP establishment, which is trying to tar the Breitbart News head and anyone associated with him as unelectable extremists.

Still, being an unequivocal supporter of Trump can be a huge boon in a Republican primary, where voters have weighed a candidate’s willingness to support Trump as a major factor in their decision-making. And Heller’s public criticism of Trump during last year’s presidential race won’t help.

“Loyalty to Trump is now being seen not just as a litmus test, but a conservative credential over actual conservative policy,” said a national GOP strategist who is following the race.

“Plus, someone like Bannon’s involvement I think helps in a primary. It sort of serves as a stamp of approval for who has the president’s support, even if the president doesn’t formally endorse,” a Republican political consultant based in the West told The Hill.

Heller faces a tough climb with both primary and general election voters.

He’s been dogged by his refusal to publicly endorse Trump during the 2016 campaign. That’s bad news for his reelection chances, as then-Rep. Joe Heck (R-Nev.) saw for himself when he lost his polling lead after he broke with Trump last year. At the time, Heller applauded Heck’s decision.

Heller remained mum on his 2016 presidential vote, only revealing this August that he voted for Trump in the face of worsening primary chances.

Heller’s public waffling on health care appears to have hurt his standing with voters on both sides of the aisle. Heller faces the worst of both worlds on the botched ObamaCare repeal, since he came out forcefully against the GOP’s plan before ultimately voting for a slimmer version.

Heller supporters argue that his more moderate brand of Republicanism will give the GOP a better shot at holding the seat in 2018, a move to the center they say is necessary after Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE won the state in 2016.

But even if Heller survives his primary, he’ll likely face a formidable Democratic challenger in Rep. Jacky Rosen (Nev.) during the general election. Along with Clinton’s 2016 win, Democrats won Nevada’s Senate seat in 2016. Morning Consult’s most recent polling showed Heller holding the seventh-highest disapproval rating among incumbent senators.

Tarkanian is a familiar face for Republican voters, thanks in part to his five failed bids for office. The son of legendary college basketball coach Jerry Tarkanian, he ran for a Senate seat in 2010, only to come in third in the primary. His most recent run for office was for an open Nevada House seat, which he lost to Rosen in the general election by 1 point.

Tarkanian insisted to The Hill that his belief in Trump’s agenda would be his ticket to victory over Heller, both in the primary and the general election.

“There are some people who are turned off by President Trump — you see the negative ratings more so because of the way he’s been attacked so much in the media, and some of the things he’s done on a personal basis. But if you ask people, ‘Do you like the kinds of things he’s doing?’ most people do,” Tarkanian said.

“What [voters] are going to care about most is: Did you support Donald Trump when he needed it the most or did you run and repudiate him?”

Heller’s campaign did not respond to a request for comment.

Heller has sought to frame Tarkanian as unreliable, pointing to an August BuzzFeed News report where Tarkanian said he would back Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE (R-Ky.), a commitment he’s since gone back on. The campaign’s press releases also jab Tarkanian on his repeated unsuccessful bids for office, declaring him a “perennial candidate.”

Tarkanian told The Hill that he doesn’t expect this to be a problem. He pointed to his success defeating two well-known state Republican lawmakers in previous primaries, as well as his tough race against Rosen, as proof that his repeated unsuccessful campaigns are actually a positive.

“The fact that I’ve lost five times ... is something that on the outside looking in is going to cause pause for some people — not the voters as much as the big donors, which is why I think Heller is doing it,” he said.

Tarkanian has led Heller in each of the last two public polls from JMC Analytics — in August by 8 points and in late October by 6 points. Heller’s team counters with its own internal poll, commissioned by the campaign and the National Republican Senatorial Committee, that found him up by 22 points in August.

As the race heats up, both candidates have attracted prominent allies. The Republican establishment — including the Senate Leadership Fund, the super PAC aligned with McConnell — is ready to go to battle for Heller. In the other corner, Tarkanian is expected to win an endorsement from Bannon’s allies at the Great America Alliance, a pro-Trump outside group.

Aggressive sparring last week between the Senate Leadership Fund and Tarkanian highlighted the group’s scorched-earth attempt to discredit Bannon and any candidate he backs, as well as Tarkanian’s allegiances.

After needling Heller by tweeting a pledge to oppose McConnell remaining as majority leader, the Senate Leadership Fund responded by asking Tarkanian if he’d pledge to back Bannon over accusations by his ex-wife, stemming from their divorce, that Bannon made anti-Semitic comments while looking at schools for his daughters.

Bannon’s adviser framed the comments as “desperate and false.” And Tarkanian’s campaign issued a comment blasting the Senate Leadership Fund and McConnell while defending Bannon.

While Heller hasn’t responded to questions about whether he backs McConnell’s leadership, he’s attempted to tie himself strategically to Trump. Heller joined with eight Republicans in an October letter that called for McConnell to “turn the Senate on full time, 24/7, to advance the president’s agenda.”

Bannon’s presence cuts both ways for Tarkanian, even as he remains publicly on the sidelines. He’s a valuable signal to galvanize Trump supporters that gives Tarkanian a shot in the arm against an incumbent that will likely outspend him. But Bannon could also alienate more moderate Republican voters, and the association could continue into the general.

“I don’t think we’ve seen any indication that his ability to connect with general election voters is any better [than previous races],” the Western-based political consultant said of Tarkanian. “The Trump-Bannon-Tarkanian triumvirate is ripe to be demonized in a general election in Nevada.”