WELLINGTON, Ohio—For three decades, Rob Portman checked all the boxes of a classic pro-business Ohio Republican: Senior positions in both Bush White Houses. Twelve years in the House. A short stint as a lobbyist. A dozen years in the Senate.
Until recently, Portman-style candidates typified the party in the state, fending off opposition from socially conservative or tea party activists. No more. With Mr. Portman retiring, the fight to succeed him revolves around one question only: Which flavor of Trump is best?
Five of the six contenders in next May’s GOP primary offer slightly different variations on the former president’s persona to voters—as well as to Mr. Trump himself. All have made pilgrimages to his South Florida estate seeking an endorsement.
The lineup shows how former President Donald Trump has only enhanced his influence among Republicans in the eight months since he grudgingly left office, a few days before Mr. Portman said he wouldn’t seek re-election.
The Ohio contest is one of a handful likely to determine control of the Senate, and what happens there could be a leading indicator of the viability of Trumpism without Mr. Trump on the ballot. The outcome will provide essential data points on Mr. Trump’s own decision about whether to run for president again in 2024 and what it will mean if he does.
“I’m watching Ohio very, very closely,” Mr. Trump said in an interview. “They’re all for Trump—it’s a wonderful thing.”
It’s unclear whether it’s also a wonderful thing for the party, in particular whether a strategy of devotion to Mr. Trump will work on a state level in a vote two years after it failed nationwide in the presidential election. Polls show a majority of Americans have an unfavorable view of Mr. Trump, which was also true throughout his presidency.
Ohio has been a presidential battleground for two decades, yet Mr. Trump won the state by more than 8 percentage points in both of his races. Ohio Republicans have won 16 of 17 statewide contests held during the past three midterm elections. Their single loss was to Democratic Sen. Sherrod Brown, whose blue-collar, economic populism—which also appeals to many of Mr. Trump’s working-class supporters—is embraced by the front-runner for the Democratic nomination for Mr. Portman’s seat, Rep. Tim Ryan.
“Democrats can still win here, especially if the Republican nominee is nothing but a Trumper,” said Aaron Pickrell, who oversaw former President Barack Obama’s 2008 victory in the state.
Mr. Trump has long delighted in displays of devotion and demanded them from those seeking his endorsement. His distinctive brand of politics has proved impossible to duplicate in its entirety. The result has been a reordering of Republican primary battlefields across the country. Conservative candidates have placed bets on which tentacle of Trumpism is both powerful in a primary and can sustain itself in a general election.
In Ohio, investment banker Mike Gibbons pitches himself as the ultimate Trump delegate, leaning into his white, working-class upbringing to appeal to the core of the Trump political base.
Josh Mandel, a former state treasurer, has adopted an aggressive social-media presence to match Mr. Trump’s and to reframe his 20 years in state politics for voters who prefer an antiestablishment candidate.
Businessman Bernie Moreno draws parallels between private-sector business careers that paid off for both him and Mr. Trump.
Jane Timken, a former state party chairwoman, promotes herself as a Trumpian field general whose years in the political trenches optimized her ability to build coalitions and win general elections.
And J.D. Vance, a venture capitalist and author, has positioned himself as the intellectual heart of Trumpism during repeated media appearances with two custodians of that mantle: Tucker Carlson of Fox News and the former Trump strategist Steve Bannon, who hosts a conservative podcast.
The one exception is Matt Dolan, a state senator and former prosecutor, who entered the race Sept. 20 as a traditional pro-business candidate and not a Trump acolyte, prompting an immediate response from the ex-president.
“I know of at least one person in the race who I won’t be endorsing,” Mr. Trump said in a written statement the same day.
Though Mr. Trump hasn’t yet endorsed a candidate in Ohio, he told the Journal he is impressed by the “very good candidates in line with my thinking.” In his view, “The single biggest issue is the election fraud of 2020,” he added. There was no evidence of widespread fraud in the 2020 election.
Mr. Trump has endorsed more than three dozen candidates in the 10 months since he lost re-election, an effort at political influence by an ex-president without parallel in the past century. Not since Teddy Roosevelt regretted handing the reins of the Republican establishment to William Howard Taft in 1908 has a former president enmeshed himself so thoroughly in party politics.
By keeping the party under his thumb, Mr. Trump is maintaining his relevance for a potential rematch with President Biden. Mr. Trump has privately described his concern that his prominence as party kingmaker would fade if he stopped tending to his supporters, said people who have spoken to him.
Candidates have lined up outside his door. Some have scheduled meetings by having members of Congress reach out to Mr. Trump. A handful of candidates have reported spending a combined $119,600 to hold fundraisers at his golf clubs in Florida and New Jersey.
Silicon Valley billionaire Peter Thiel has coordinated private meetings with Mr. Trump for two Senate candidates the PayPal co-founder is backing—Mr. Vance in Ohio and Blake Masters in Arizona. Mr. Thiel has put $10 million into each race but said at a Sept. 10 meeting with Mr. Trump and Mr. Masters that the former president’s endorsement was more valuable, according to people briefed on the meeting.
Many of Mr. Trump’s endorsements so far appear guided by his appetite for political payback, of both the benevolent and vengeful variety. About half have gone to current officeholders he views as allies. They have generally supported his continued bid to contest the 2020 vote and are expected to win re-election in 2022.
He has fought with his own advisers over endorsements in some cases, according to people briefed on the discussions. They cited his backing of Vito Fossella for Staten Island borough president in New York City.
Mr. Fossella was a five-term congressman in 2008 when a scandal over having two families derailed his career. The Trump relationship dated back to Mr. Fossella’s time as a New York City councilman, and in June Mr. Trump was eager to help despite advisers’ doubts Mr. Fossella could win the nomination.
“Every politician was against him in New York—but Vito had me,” Mr. Trump said. “There’s something good about loyalty in politics, and you don’t see too much of it, unfortunately.” Mr. Fossella took the nomination in July and faces the general election next month.
Other Trump endorsements aim at unseating two types of Republicans: election officials in 2020 battleground states he lost but contends he won; and federal lawmakers who backed an impeachment charge related to the Jan. 6 Capitol riot. Ten Republicans voted yes when the House impeached Mr. Trump in January, before the Senate acquitted him.
The strategy has already paid dividends. On Sept. 16, Republican Rep. Anthony Gonzalez, who voted for that second impeachment of Mr. Trump, folded his re-election bid in suburban Cleveland, citing “the toxic dynamics inside our own party.”
Mr. Trump’s most frequent endorsements have been in Senate races, where he has backed 12 candidates. Only the one in Georgia—of Herschel Walker, a former football star and longtime family friend—involves a seat currently held by Democrats.
To retake the Senate, Republicans need to pick up one of 14 Democratic-held seats on the ballot next year and defend 20 Republican-held ones. Among those are five seats where their incumbents are retiring. Mr. Trump has endorsed candidates for those in Alabama, North Carolina and Pennsylvania but not in Missouri or Ohio.
In Ohio, the quintet of pro-Trump Senate candidates are scrubbing past criticism of Mr. Trump from social media, hiring former Trump campaign officials and seeking endorsements from former officials of his administration.
Each has the means to assemble a fully loaded war chest for the primary battle. In interviews, all five said they expected to raise—from individual contributions or their own bank accounts—the $10 million that political operatives estimate a successful primary contest would cost.
That would give them all the ability to pitch to the two million Republican voters in the state’s 12 media markets. A primary with so many well-funded candidates would be unprecedented, political operatives said.
An early fundraising surprise is Mr. Moreno, a 54-year-old car dealer and tech entrepreneur who raised $2.2 million in the first three months of his campaign, roughly double what anyone else in the race collected in individual contributions.
Mr. Moreno’s campaign trades on his only-in-America biography. He emigrated from Colombia as a boy, took big risks in the car business and now is worth tens of millions of dollars, with properties in Cleveland, Columbus, Washington, New York and the Bahamas.
In another era, his biggest liability might be his relatively short 16 years living in the state. Instead, it is having referred to Mr. Trump in 2016 as a “lunatic invading the party.”
Mr. Moreno says he changed his view because of how Mr. Trump pushed to deliver on his campaign promises. “One of the greatest presidents I’ve ever seen,” he said at a fundraiser.
Mr. Moreno has hired Kellyanne Conway, the 2016 Trump campaign manager. A daughter, Emily, was a Republican Party official in 2020 and is engaged to Max Miller, a Trump aide who was Mr. Trump’s choice to unseat Mr. Gonzalez in the Cleveland-based House race.
Mr. Gibbons, the investment banker, who has put $5.7 million into the race, is also running on his business experience, but in a way that mirrors more closely Mr. Trump’s unpolished approach. In an interview, Mr. Gibbons, 69, referred to the 43-year-old Mr. Mandel as “a boy.”
Mr. Gibbons said there was more election fraud in Mr. Trump’s loss than in his victory four years earlier, but wasn’t willing to say the 2020 contest was stolen.
Mr. Gibbons said Trump supporters who committed crimes on Jan. 6 should be prosecuted, but he also said descriptions of the violence that day were overblown.
Mr. Gibbons wants Mr. Trump to stay out of the race. He said he saw how much of a game-changer Mr. Trump’s involvement had been in 2018 when Mr. Gibbons was seeking the GOP Senate nomination that year.
When the 2018 front-runner, Mr. Mandel, dropped out, Mr. Trump recruited a congressman to run against Mr. Gibbons in the primary, even though Mr. Gibbons had been a co-chairman of Mr. Trump’s 2016 fundraising efforts in Ohio. The person Mr. Trump recruited, Rep. Jim Renacci, got the nomination but lost the general election.
Combined with the candidates’ ample funding, the absence of a Trump endorsement has kept the playing field level—an advantage for Mr. Mandel, who entered the race as the best known.
Mr. Mandel, a suburban Cleveland council member at 26, is mounting his third bid for the Senate. Polls show Ohio Republicans generally like him, but few love him. He lost to Sen. Brown in 2012 and quit the 2018 primary race despite being ahead. Mr. Trump has pressed him about his commitment to the current contest.
At a June rally in Ohio, Mr. Trump polled his audience about the contenders. When Mr. Mandel got the warmest reception, Mr. Trump said, “I think we’ll get out of this poll stuff.”
Mr. Mandel has done his best Trump impression with his social-media presence. Allies and opponents alike privately speculated he was trying to get himself banned from the same platforms that have barred Mr. Trump.
In an interview, Mr. Mandel said Republican voters were looking for brawlers to represent them in Washington and explained his combative posts by saying he has been liberated by the unapologetic approach of Trumpism. He said he believed Mr. Trump got more votes than Mr. Biden in 2020, without providing evidence.
”Now is not the time for bipartisanship, now is not the time for civility,” Mr. Mandel said. “Now is the time for fighters.”
Mr. Mandel’s attempt to secure Mr. Trump’s endorsement has been largely defined by his bid to keep it away from opponents, advisers said. He has attacked perceived front-runners for the backing by highlighting times he says they have been out of step with Mr. Trump.
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Mr. Mandel’s latest focus is Mr. Vance, who has started to rise in candidates’ internal polls.
Mr. Vance’s 2016 bestselling memoir, “Hillbilly Elegy,” was about growing up in southwest Ohio but was also a cultural critique of white, working-class America. As a candidate, he has pushed for policies outside of the conservative mainstream, including a plan that would raise taxes on corporations that move jobs overseas.
Mr. Mandel has attacked him as a “Never Trumper” and promoted video clips from 2016 showing Mr. Vance referring to some Trump voters as racists. Mr. Vance also referred to the party’s standard-bearer as “cultural heroin” in 2016, and he has deleted tweets critical of Mr. Trump, including one that called him “reprehensible.”
Mr. Vance, 37, said in an interview he was wrong about the ex-president. “He fought a lot of very powerful institutions that needed to be fought against,” Mr. Vance said. “The president opened a lot of doors for people who think the way I think on trade, immigration and foreign policy.” He conveyed a similar message to Mr. Trump when the two met earlier this year and asked him to keep an open mind in the race, said people familiar with the conversation.
A super-PAC through which Mr. Thiel is backing Mr. Vance’s campaign hired former Trump advisers, including pollster Tony Fabrizio and political strategist Andy Surabian. Mr. Surabian has moved over to the Vance campaign as a senior adviser.
Mr. Mandel’s first target was Ms. Timken, a 55-year-old lawyer with experience in civil litigation and business disputes. She became chairwoman of the state Republican Party in 2017 when Mr. Trump helped her oust a chairman he deemed insufficiently supportive.
Trump advisers said the president was on a path to backing Ms. Timken for Senate bid but reconsidered after remarks she made after Rep. Gonzalez’s January vote to impeach. In an interview with the Plain Dealer, Ms. Timken called Mr. Gonzalez “an effective legislator” and a “very good person” who had “a rational reason” for his vote.
Two weeks later, she launched her Senate campaign. Among her first news releases was a statement calling on Mr. Gonzalez to resign.
In an interview, Ms. Timken described herself as the most vocal Republican in the race against both impeachments of Mr. Trump. Asked if she believed there was widespread voter fraud in the 2020 elections, she said that “there were shenanigans.”
“Going forward, we need to make sure we secure our elections so that people have faith in our election system,” she said. “That’s all I’ll say about that.”
Write to Michael C. Bender at Mike.Bender@wsj.com.
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