“America’s Hitler.” “A total fraud.” “A moral disaster.” Those were a few of the descriptions that JD Vance, bestselling author of the memoir Hillbilly Elegy, once offered for Donald Trump. But none of that past criticism was evident here last Saturday night, as Vance shared a stage with Trump to accept the former president’s endorsement in the Ohio Senate Republican primary.

“He’s the guy that said some bad shit about me,” Trump said of Vance during his rally at the Delaware county fairgrounds. “If I went by that standard, I don’t think I would have ever endorsed anybody in the country.”

Taking the podium as the rally crowd chanted his name, Vance said, “The president is right. I wasn’t always nice, but the simple fact is, he’s the best president of my lifetime, and he revealed the corruption in this country like nobody else.”

In less than six years, Vance has gone from a Trump skeptic who openly contemplated voting for Hillary Clinton to a devoted loyalist who has endorsed finishing the border wall and denounced identity politics as a Democratic gimmick. Vance’s radical shift reflects the larger transformation of the Republican party, as it has become nearly impossible to succeed in a primary as a Trump critic.

The stakes are high for Republicans in Ohio, as they try to hold on to retiring Senator Rob Portman’s seat and ultimately regain control of the evenly divided upper chamber of Congress. The Republican candidate who wins the 3 May primary will probably face off against Democratic congressman Tim Ryan in the general election. The Republican primary winner, whoever it is, will then be favored to win in November, considering Trump defeated Joe Biden in Ohio by eight points in 2020.

For Trump, the Ohio Senate election also represents the biggest test yet for his most influential tool as party leader: a primary endorsement. Trump’s endorsement of Vance was considered a gamble, given that he had been trailing in the limited public polling of the race. Before Trump’s announcement, former state treasurer Josh Mandel and businessman Mike Gibbons were widely considered the two frontrunners in the primary. But a new poll released this week showed Vance pulling ahead of Mandel and Gibbons.

“Before the Trump endorsement, I think JD was probably top of the second tier behind Gibbons and Mandel,” said Mike Hartley, who previously served as a senior adviser to former Ohio governor and Republican presidential candidate John Kasich. “I believe that the former president’s endorsement has made this race an actual toss-up.”

Now Vance is counting on Trump’s endorsement to carry him to victory. And Trump is counting on that, too.

“Meanness and pettiness and just plain craziness”

When Vance announced his Senate candidacy last year, he began his campaign with a remorse tour, seizing every opportunity to explain his change of heart about Trump.

“I ask folks not to judge me based on what I said in 2016,” Vance told Fox News last July. “I’ve been very open that I did say those critical things and I regret them, and I regret being wrong about the guy.”

Vance’s groveling was successful in convincing the person whose opinion mattered most in the race: Trump. The former president announced earlier this month that he would endorse Vance, even though several candidates, including Mandel and Gibbons, had been openly campaigning for Trump’s support.

“Like some others, JD Vance may have said some not so great things about me in the past, but he gets it now, and I have seen that in spades,” Trump said in his endorsement. “He is our best chance for victory in what could be a very tough race.”

But Vance’s critics have not been so easily convinced. As reports emerged of the likely endorsement, Ohio Republican leaders who had backed other candidates circulated a letter urging Trump to reconsider. The letter noted that Vance had previously attacked some of Trump’s supporters as racist and chose to support independent candidate Evan McMullin in the 2016 presidential election, after he considered voting for Clinton.

“I don’t have a problem with somebody changing their mind about somebody, and JD Vance has expressed on any number of occasions that he changed his mind as Trump held office,” Gibbons told the Guardian during an event with supporters in Powell, Ohio, last week. “What troubles me is that he actually kind of played with voting for Hillary Clinton. And I don’t think you can have a shred of conservative ideology to think for one second about voting for Hillary Clinton.”

Despite the last-minute effort to prevent the endorsement, Trump went ahead with his plans to publicly back Vance. Rather than inspiring unity among Ohio Republicans, the endorsement appears to have injected even more vitriol into a race that was already defined by, as one columnist said, “meanness and pettiness and just plain craziness”.

The conservative group Club for Growth, which has thrown its support behind Mandel in the race, has continued to air ads highlighting Vance’s past negative comments about Trump. This week, the group released a new ad arguing Trump had made the wrong choice in backing Vance and highlighting the former president’s 2012 endorsement of Mitt Romney, who lost to Barack Obama.

When Trump learned that Club for Growth, with whom he has worked in the past, was standing by its endorsement of Mandel, he reportedly had his assistant text the group’s president, “Go fuck yourself.”

Trump’s closest allies have engaged in similar mudslinging, intent on proving that the former president’s endorsement is enough to guarantee victory in a close race. Donald Trump Jr, the former president’s eldest son, who has campaigned with Vance in recent days, attacked Mandel on Twitter as the “Club for Chinese Growth backed establishment candidate”.

Both Mandel and Vance have the backing of super PACs that have spent millions of dollars to aid their campaigns, making it even easier to flood the airwaves with attack ads. The pro-Vance group Protect Ohio Values has been propped up by $13.5m in donations from the tech investor Peter Thiel, who reportedly lobbied Trump directly for his endorsement. Meanwhile, Gibbons has mostly self-funded his run, lending more than $16m to his campaign.

The cost of the primary has given Ohio Democrats, who have relished the ugliness of the Republican race, another opening to criticize the entire field of candidates.

“It was already a race to the bottom, and Trump’s visit will make it a photo-finish to the gutter for this nasty, chaotic and expensive primary,” Elizabeth Walters, chair of the Ohio Democratic party, said ahead of the former president’s rally. “No matter which one of them hobbles out of the primary, we are ready to fight back and show working families that we’re on their side.”

“I don’t have to follow Trump’s every whim”

Vance is hoping that Trump’s endorsement will be enough to ensure he is the candidate who “hobbles out of the primary”, and there are indeed signs that he has picked up support. A Fox News poll taken after Trump’s announcement showed Vance leading Mandel by five points, although that advantage is within the survey’s margin of error. Gibbons is now trailing Vance by 10 points, the poll found.

“The endorsement’s already given us a ton of momentum,” Vance told reporters after his town hall last week in Huber Heights, just outside Dayton. “It’s my race to lose, but at the end of the day, we still have to do the work.”

At Vance’s town hall, several attendees said Trump’s endorsement had brought their attention to Vance in a new way. Judy Coeling, a 59-year-old primary voter from Centerville, said she had previously been deciding between Mandel and Gibbons, but Trump’s endorsement had prompted her to reconsider Vance.

“I came just to find out more information because I had two other people in mind that I was kind of debating,” Coeling said. “I think he’s probably a lot stronger than what I gave him credit for.”

Despite his opponents’ efforts to smear him for his past criticism of Trump, some of Vance’s supporters also said that they understood his negative opinion because they also once had their doubts about the former president.

“I thought Trump had no chance, and then I saw how skilled he was,” said Evron Colhoun, a retiree from Englewood who volunteers with the Vance campaign. “I would say that I identified with that, too, because we had a similar evolution [with Trump]. I mean, I was really surprised that Vance got his endorsement, but I see why.”

“I get where he’s coming from,” Brian Kitchen, a 48-year-old voter from Huber Heights, said of Vance’s past anti-Trump views. “I was a Kasich supporter, so I still have a Kasich bumper sticker on my other car.”

Vance’s opponents have attempted to downplay the impact of the endorsement, claiming they have not seen a significant decline in their support. “I literally have gotten – and I’m not exaggerating – hundreds of texts saying, ‘We’re with you,’” Gibbons said. “I’ve only had one person that said they were switching their vote.”

Standing outside an early voting site in Columbus last week, Drew Sample, 37, said he had still voted for Gibbons because of his misgivings about Vance. “I don’t completely dislike him. I just don’t think he’s qualified to be senator,” Sample said. “He’s an opportunist. I think most of these guys are opportunists.”

At the Trump rally in Delaware, some of the former president’s most ardent admirers were not persuaded by his endorsement of Vance. Jessica Dicken, a 31-year-old voter from Logan, said she would instead be supporting Mark Pukita, who has bragged about being unvaccinated and was accused of making an antisemitic ad about Mandel. (He denied that charge.)

“I kind of think for myself,” Dicken said as she lined up hours early to enter the rally grounds. “I definitely support Trump, but I’m going to vote for my own Ohio primary who I feel is fit and who’s best.”

Laura Beringer, another Pukita supporter from Akron standing a bit behind Dicken in line, added, “I love Trump, but he’s not perfect. I don’t have to follow his every whim. I have to go by what I feel.”

Democrats are hoping they will be able to capitalize on those internal divisions in November and flip Portman’s seat. “Trump’s visit won’t unite the party,” Walters said. “All it really does is throw more fuel on the dumpster fire that is the Senate and gubernatorial races in Ohio on the Republican side.”

But Republicans in Ohio – and across the country – have a number of factors working in their favor for November. The president’s party traditionally loses seats in Congress during the midterm elections, and Biden’s approval rating continues to languish in the low 40s. The Cook Political Report has rated the Ohio Senate race as “lean Republican”.

“I think any of the [top candidates] will beat Tim Ryan,” Hartley said. “With this political environment, if a Republican doesn’t win, it’s going to be their own damn fault.”