On Tuesday, voters in Indiana and Ohio will go to the polls to vote in primary elections for everything from Congress to county executive. But there’s one race in particular that everyone is watching: Ohio’s Republican primary for U.S. Senate. (Don’t worry, though, we’ll be back tomorrow with a preview of the other key races to watch!)

This contest has gotten a lot of attention for a couple of reasons. First, it’s an open seat — Sen. Rob Portman is retiring — and the election to replace him is wide open. Of the seven Republicans running, four or five have a legitimate shot at the GOP nomination, and the winner of the GOP nomination will be a heavy favorite to become Ohio’s next senator.

Second, the primary is a chance for voters to nominate a very different kind of Republican from Portman, who served in the administrations of Presidents George H.W. Bush and George W. Bush and was more willing than most members of his party to criticize former President Donald Trump. But now loyalty to Trump has become a key litmus test in the race to replace Portman. For example, all but one of the GOP primary candidates have embraced Trump’s “Big Lie” that the 2020 election was illegitimate.

True, Ohio’s junior senator is likely about to get Trumpier, but the candidates still fit different molds within the GOP. And even though Trump has now issued one of his coveted endorsements in the race, other factions of the party haven’t given up the fight either — which means the primary will be a direct test of how much clout Trump has when other Republican elites dare to defy him.

The race started out with a clear front-runner: former state Treasurer Josh Mandel, who at just 44 has already had a long career in Ohio politics, serving in the state legislature and running for U.S. Senate twice before. (He lost to Democratic Sen. Sherrod Brown in 2012 and dropped out of the 2018 race, citing a health issue with his then-wife.) Once a suburban moderate, Mandel has spent this Senate campaign embracing right-wing culture wars and over-the-top rhetoric, becoming a favorite of ideological purists in the conservative movement like Texas Sen. Ted Cruz and the anti-tax Club for Growth, both of which have endorsed him.

However, Mandel struggled to lock down the nomination. For much of 2021, the other candidates released dueling internal polls arguing that they were the most viable alternative to Mandel, whom many Ohioans view as a ladder-climbing opportunist for the way he has glommed on to the Trump wing of the party.

Former Ohio GOP Chair Jane Timken was the first candidate to pose a threat after Trump reportedly came close to backing her early last year; however, after Mandel’s allies highlighted some of her more establishment tendencies (she defended a Republican representative who voted to impeach Trump in 2021), the endorsement never came through, and she faded back into the pack. (One notable Republican — Portman — did eventually endorse Timken, but this doesn’t seem to have buoyed her campaign. She’s in fifth place in most recent surveys.)

That left an opening for political outsider Mike Gibbons, who has the backing of Kentucky Sen. Rand Paul. An investment banker, Gibbons has spent almost $17 million of his own money on his campaign, and it appeared to pay off in early 2022, when multiple independent polls gave him a share of first place. In March, though, he almost came to blows with Mandel at a debate and, relatedly or not, stopped rising in the polls shortly thereafter.

More recently, state Sen. Matt Dolan has experienced a small polling surge, even pulling into a virtual tie for the lead in one (albeit with only 18 percent). The wealthy Dolan, part-owner of the Cleveland Guardians baseball team, has donated or loaned his campaign more than $10 million, but he has a significant liability that could cap his support: He is the only candidate who has been willing to break with Trump, saying the 2020 election wasn’t stolen and condemning the Jan. 6 Capitol riot. Dolan has made a point of saying that he’s not actually anti-Trump, but for his part, Trump is certainly anti-Dolan: The former president has attacked Dolan for the Guardians’ decision to shed its racist former name, the Indians (a decision Dolan says he actually opposed).

So, the candidate who appears to have the strongest momentum of late is J.D. Vance, author of the best-selling book “Hillbilly Elegy.” Vance jumped into the race with much fanfare — including a $10 million donation from PayPal co-founder Peter Thiel to a pro-Vance PAC — but for much of his campaign, he was hammered by TV ads highlighting his opposition to Trump in the 2016 election and languished around 10 percent in the polls. But like Mandel, he has plunged head-on into conservative grievance politics to ingratiate himself with Trump and his supporters, and unlike Mandel, it has worked: On April 15, Trump bestowed his coveted endorsement on Vance, and he has since shot up in the polls. The most recent nonpartisan poll of the race, conducted April 20-24 by Fox News, gave Vance 23 percent, Mandel 18 percent, Gibbons 13 percent, Dolan 11 percent and Timken 6 percent, with 25 percent still undecided.

As such, the final days of the campaign have devolved into open warfare between Trump and the pro-Mandel Club for Growth. The two have long been frenemies: The group opposed Trump in 2016, made nice with him during his presidency, but is now unafraid to defy him when necessary. For instance, shortly after Trump endorsed Vance, the Club for Growth kept airing a TV ad highlighting his past anti-Trump comments. When Trump reportedly had an aide text the group’s president to “go f*^% yourself,” the Club for Growth responded by increasing the ad buy.

The Republican primary for Ohio Senate will likely be seen as the first real test of Trump’s grip on Republican voters since leaving office. But while a Vance loss would certainly be bad news for Trump, it wouldn’t exactly be a repudiation of him, considering that much of the argument against Vance has focused on the fact that he himself used to be anti-Trump. Moreover, a Sen. Mandel, Gibbons or Timken would still (perhaps to varying degrees) stand shoulder to shoulder with Trump in the Senate. Unless Dolan sneaks up the middle, Republicans’ Senate candidate will still be a much stronger defender of Trump than Portman was.

Of course, the Republican nominee will still have to beat the winner of the Democratic primary before reaching the Senate. And Democrats have their own intraparty squabble for this seat. Moderate Rep. Tim Ryan is backed by establishment figures like Brown and former Secretary of State Hillary Rodham Clinton and has wide leads in the few Democratic primary polls that have been released. But Ryan has also angered the left wing of the party with recent campaign ads in which he vilifies China for the plight of American workers, and at least one progressive group has endorsed former Consumer Financial Protection Bureau senior adviser Morgan Harper (though others have notably kept their powder dry).

Ryan’s brand as a blue-collar populist is often touted as Democrats’ best hope to score only their second statewide victory in Ohio since 2012. But Ryan did just 1.5 percentage points better than President Biden in his district in the 2020 election, and Biden lost Ohio by 8 points. Given that Ohio has become a Republican-leaning state and 2022 is shaping up to be a Republican-leaning year, the winner of the Republican primary on Tuesday is very likely going to be the state’s next U.S. senator.