Former President Donald Trump's rally for Ohio Senate candidate J.D. Vance Saturday will likely be the biggest event of the hotly-contested race — and the race may be the biggest test of Trump's endorsement power yet. 

Trump backed Vance for the GOP Senate nomination in Ohio after months of the top candidates jockeying for his support. Rivals Mike Gibbons, Jane Timken and Josh Mandel — all advised and endorsed by at least some Trump-world regulars — sparred with Vance and each other about whose support for Trump was more genuine. 

For most of the race it appeared Mandel and Gibbons were the frontrunners. They were effectively tied atop a Fox News Poll of the race conducted in early March, with Vance, Timken and Ohio state Sen. Matt Dolan all clustered in a second tier in the race. 

Republican Ohio Senate candidate J.D. Vance at the "Up from Chaos" conference hosted by the American Moment nonprofit and The American Conservative magazine. (Fox News)

Republican Ohio Senate candidate J.D. Vance at the "Up from Chaos" conference hosted by the American Moment nonprofit and The American Conservative magazine. (Fox News)

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But then Trump picked Vance, putting Vance squarely in contention for the nomination. 

"Here is a case where the Trump endorsement could really make all the difference," University of Virginia Center for Politics Director Larry Sabato told Fox News. "He wasn't one of the two leaders. But it seems to me like Trump's endorsement could easily propel Vance to a victory." 

Sabato added: "There's no runoff here. And if you've got five candidates splitting up the vote, it becomes easier. And if this was part of Trump's calculation, it was a smart calculation."

From Vance's perspective, the Trump endorsement takes the sting out of one of the most common attacks against his candidacy: That he was highly critical of Trump during his first run for president in 2016.

Former President Donald Trump holds a rally sponsored by Save America with Ted Budd, Madison Cawthorn, Bo Hines, Dan Bishop, Mark Robinson and Greg Murphy in Selma, North Carolina, on April 9, 2022. (Photo by Peter Zay/Anadolu Agency via Getty Images)

Former President Donald Trump holds a rally in Selma, North Carolina, on April 9, 2022. (Peter Zay/Anadolu Agency via Getty Images)

"It's hugely powerful. I think it's the only endorsement in politics that actually moves votes, and it moves a lot of votes, especially here in Ohio," Vance told Fox News of Trump's endorsement. 

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"Especially given so many of the attack ads against me have centered around the fact that I was critical of Trump in 2016, yes, it's pretty solid reinforcement that the president trusts me," Vance added. "Obviously, the president is endorsing me and sees me as the candidate who is most aligned with his agenda. And so I think it defeats a lot of the negative attack ads against my candidacy."

Vance told Fox News that attendees at the Trump rally can expect to be treated to not only a reinforcement of support from the former president, but also the traditional Trump rally entertainment. 

"We'll have a great speech from Trump where half of it will be scripted and half of it will be, you know, him telling jokes and making everybody have a good time," Vance said. 

JD Vance, at the Conservative Political Action Conference (CPAC), speaks with Fox News, on Feb. 26, 2022 in Orlando, Florida.

JD Vance, at the Conservative Political Action Conference (CPAC), speaks with Fox News, on Feb. 26, 2022 in Orlando, Florida. (Fox News )

The Ohio GOP race will be a notable test for Trump's endorsement power largely because of the fact so many candidates are trying to align with Trump, and because it's so early. 

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Other races with similarly crowded fields of pro-Trump candidates haven't happened yet — including the Pennsylvania and Alabama Senate races, and the Nevada governor's race. 

Other cases for post-presidency Trump endorsements include Virginia Gov. Glenn Youngkin — which was in a general election that focused on schools – and the Texas GOP primaries with entrenched incumbents like Gov. Greg Abbott. 

Tedd Cruz CPAC

Sen. Ted Cruz, R-Texas, speaks at the Conservative Political Action Conference (CPAC) Thursday, Feb. 24, 2022, in Orlando, Fla. Cruz will campaign for Ohio Senate candidate Josh Mandel ahead of the GOP primary. (AP Photo/John Raoux)

Plus, in Ohio even the candidates Trump didn't endorse embrace "Trumpism" and have credible support from Trump's Republican allies. 

Gibbons is supported by Rep. Kat Cammack, R-Fla., and Sen. Rand Paul, R-Ky.

Timken is backed by Sens. Rob Portman, R-Ohio, Shelley Moore Capito, R-W.Va., Deb Fisher, R-Neb., and Joni Ernst, R-Iowa. 

Sen. Ted Cruz, R-Texas, supports Mandel and will be campaigning with him around Ohio in the final days before the primary election on May 3. 

Dolan, who was the only major candidate in the race not lobbying hard for Trump's backing, earned the endorsement of the Cleveland Plain Dealer earlier this month. 

GOP Senate candidate Mike Gibbons of Ohio speaks with Fox News at the Conservative Political Action Conference (CPAC), on Feb. 26, 2022 in Orlando, Florida 

GOP Senate candidate Mike Gibbons of Ohio speaks with Fox News at the Conservative Political Action Conference (CPAC), on Feb. 26, 2022 in Orlando, Florida  (Fox News )

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"America First is more than just the Republican Party or a specific politician. It’s a movement that is striving to put the interests of the American people back at the core of government," Gibbons said in a statement. "Real, authentic conservative leadership is necessary to take on the failures of the Biden Administration and conservative voters take it upon themselves to decide who is most qualified to represent the party."

"Jane Timken has the proven America First record that nobody else in the race can claim," Timken spokeswoman Mandi Merritt said. "While JD Vance was in Silicon Valley and cable news studios calling President Trump ‘America’s Hitler’ and bashing the MAGA movement as ‘racist,’ Jane Timken was traveling 150,000 miles across Ohio doing the important work to dismantle the Kasich establishment and turn Ohio into a pro-Trump, conservative stronghold."

Jane Timken

Ohio Senate candidate and former state GOP chair Jane Timken signs a 'Stop Critical Race Theory' pledge, at her campaign office in Columbus, Ohio on July 12, 2021 (Jane Timken Senate campaign)

The Trump endorsement of Vance doesn't appear to have quelled concerns about him from at least some Ohio activists. Half of Trump's 2016 Ohio RNC delegates signed a letter this week calling Trump's backing of Vance a "betrayal" and asking him to "reconsider." That followed a similar letter last week in which Ohio's GOP Central Committee and a group of county GOP chairs asked Trump not to endorse in the race at all. 

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Even if Trump's support for Vance doesn't result in a win for the "Hillbilly Elegy" author and former investment banker, Sabato said, Trump is unlikely to suffer any consequences.

"What Trump will say, and it's somewhat credible, really, is that, ‘Well, they’re all for me, or at least four of the five,'" Sabato said. "Trump has an easy way out… ‘They saw what a great job I was doing and they all fell in line and we will all get behind the nominee.’"

"Also, suppose Vance loses, but he loses within a couple points," Sabato added. "Well, that's defensible…. This is not going to reduce Trump's support among Republican voters because his preferred choice lost by two points, if that's what happens." 

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But Vance said he doesn't expect to lose, and believes Trump's support will help put him over the top. 

"A lot of people say that they believe in the America first agenda but don't actually have the president's backing and don't have the trust of the people who delivered on the America First agenda," Vance said. "A lot of people want votes so they say certain things. But I'm the only candidate who actually has the president's trust and endorsement."