By Eric Cox and Nathan Layne
WEST CHESTER, Ohio (Reuters) - Donald Trump's influence gets its first big test of the midterm election cycle on Tuesday, when Ohio Republicans pick their candidate for U.S. Senate, kicking off a series of critical nominating contests in the coming weeks.
The former president upended the Ohio race last month by endorsing author and venture capitalist J.D. Vance, turning the primary into a referendum on his sway over Republican voters as he weighs another White House run in 2024.
Polls show a tight race since Trump's endorsement, with Vance holding a narrow lead over state lawmaker Matt Dolan and former state Treasurer Josh Mandel. In a Sunday evening rally, Trump appeared to confuse two of candidates' names, telling the crowd that he backed "J.D. Mandel" to take the seat of retiring Senator Rob Portman.
"This is a test of his ability to anoint someone," said University of Cincinnati political science professor David Niven. "The endorsement has made this Vance's race to lose."
Voters will also choose a Democratic candidate for Portman's seat, a race led by Representative Tim Ryan, who ran a brief 2020 presidential campaign. Nonpartisan election officials favor Republicans' chances of winning the final Nov. 8 matchup.
Trump has not announced his candidacy for 2024 but he regularly hints at his political rallies that he intends to run for president again.
The governorship and a rematch between two Democratic rivals for a U.S. House seat are also on the ballot in Ohio on Tuesday, when voters in Indiana will also cast primary ballots.
A Trafalgar Group poll conducted April 29-May 1 showed Vance with support of 26.2% of likely Republican primary voters, followed by Dolan with 22% and Mandel with 20.8%. Prior to Trump's endorsement, polls had showed Vance trailing Mandel.
Trump has endorsed more than 150 candidates this year, of which Vance is among a dozen or so key picks. Trump's involvement will help determine whether Republicans, as expected, reverse their slim deficit in the House and also possibly take control of the Senate, which is split 50-50 with Democrats owning the tie-breaking vote.
A loss of control of either chamber would allow Republicans to block President Joe Biden's legislative agenda and also to pepper his administration with distracting and potentially politically damaging investigations.
Not all party members are falling in line behind Trump's lead. Similar to Ohio, Trump-backed candidates for U.S. Senate in Pennsylvania and North Carolina face well-funded Republican challengers. And some worry that Trump's picks, like former football star Herschel Walker in Georgia, could prove too controversial to prevail against Democrats in November, imperiling their bid for Senate control.
Vance, a former Trump critic, was not the choice of many party leaders in Ohio, and some have grumbled publicly about Trump's decision. The Club for Growth, a powerful conservative advocacy group, has aired ads bashing Vance and is sticking by its pick in the race, the unabashedly pro-Trump Mandel.
Miranda Yaver, a political science professor at Oberlin College, said a Vance loss would not necessarily point to the end of Trump's iron grip hold over the party, given that four of the five Republican candidates lobbied for Trump's endorsement.
"I don't think it's a real loss for the Trump agenda if Vance loses because it's still going to be an 'America First' candidate," Yaver said. "There are (other) people in this race that are more ideologically aligned with Trump."
The Democratic primary between progressive candidate Nina Turner and incumbent Shontel Brown for the 11th congressional district, which includes Cleveland, will be closely watched as a test of the power balance between the establishment and progressive wings of the party.
Also in focus is the Republican primary for governor, where incumbent Mike DeWine is expected to edge out three Republican challengers who are splitting the anti-DeWine vote. Trump did not endorse anyone in the race.
Still, the fact that DeWine, with five decades as a central figure in Ohio politics, is having to campaign hard for political survival underscores the extent to which Trump has upended the status quo.
"He's running for his political life after 50 years," University of Cincinnati's Niven said. "That tells you a lot."
Indiana also holds primaries on Tuesday. One race garnering attention is for a congressional district in a historically Democratic stronghold outside Chicago, with seven Republicans vying for the chance to oust freshman Democratic Representative Frank Mrvan in a race seen as having the potential to be competitive.
(Reporting by Eric Cox in West Chester, Ohio and Nathan Layne in Wilton, Connecticut; Editing by Scott Malone and Alistair Bell)