The GOP Senate primary in Ohio is heating up ahead of a debate later this month, with President Trump backing the least tested of the three major candidates — businessman Bernie Moreno — as each hopeful competes in the hopes of taking on Senator Brown in November.

The three major GOP Senate candidates, Mr. Moreno, the secretary of state, Frank LaRose, and Matt Dolan of the state senate, are set to face off in a GOP primary debate on January 22. It will mark the beginning in earnest of the primary, which will be held on March 19.

So far, the biggest issue that has emerged in the race is determining the candidate who is Mr. Trump’s favorite, and Mr. Moreno is winning on that front. On the last day before the deadline for candidates to file in the race, December 18, Mr. Trump announced he was endorsing Mr. Moreno.

“It’s time for the entire Republican Party to UNITE around Bernie’s campaign for Senate, so that we can have a BIG victory in what will be the most important Election in American History,” Mr. Trump wrote on Truth Social.

Mr. Trump joined Senator Vance, who had urged Mr. Trump to endorse Mr. Moreno. When Mr. Vance endorsed Mr. Moreno, he said, “We don’t just want career politicians.”

Mr. Moreno is originally from Colombia, and his father served as the Colombian equivalent of secretary of health. In the 1970s, his family moved to America, and he eventually got into the automobile industry and owned a number of Ohio car dealerships before selling them and pivoting to cryptocurrency in 2018.

Mr. Moreno’s campaign has already taken a page from Mr. Trump’s playbook, demanding Mr. LaRose recuse himself from election administration — one of the duties of his office — due to an “obvious conflict of interest.”

“It is critical that Mr. LaRose recuse himself from this process so Ohio voters have confidence he has not, once again, used his official office for political gain by unfairly denying his political opponents access to the ballot or seeking retribution,” Mr. Moreno’s campaign manager, David DiStefano, said in a cover letter when he submitted signatures to file for the race.

Mr. Trump’s decision to endorse a political newcomer likely threw a spanner in the works for the GOP after the party suffered in the 2022 midterms in part due to Mr. Trump endorsing untested candidates.

The managing editor of Sabato’s Crystal Ball at the University of Virginia’s Center for Politics, Kyle Kondik, tells the Sun that Mr. Trump’s endorsement is likely the “one that matters the most in the GOP primary.”

“Moreno also attracted the support of the anti-tax Club for Growth — a surprising, and I’m sure welcome, development for Moreno,” Mr. Kondik added. “I think you’d have to look at Moreno as the favorite now, even though he faces two other viable candidates.”

Mr. Kondik added that Mr. Moreno’s candidacy is more like Mr. Vance’s 2022 bid than that of any of the other candidates. He characterized Mr. Vance’s run as being victorious “despite a somewhat rocky campaign.”

“Moreno is the least proven candidate of the three for the general election — he hasn’t run before, other than a failed attempt for the Senate nomination in 2022,” Mr. Kondik says. “He doesn’t really fit the mold of the typically more experienced statewide candidates that the Ohio Republican Party has traditionally produced for top offices.”

The winner of the GOP primary in Ohio will face off against Mr. Brown in the general election. The senator has proved to be electorally resilient despite the state’s shift toward the GOP, winning re-election after his first victory in 2012.

In 2018, Mr. Brown defeated the GOP nominee, Jim Renacci, by nearly six points. When he filed for re-election in December, he laid out some themes for his campaign.

“I fought for Issue 1 so that women can have reproductive rights. My three opponents all still call for a national abortion ban,” Mr. Brown said at a December press conference. “Which side are you on? You’re on the side of, ‘I want women to make these decisions. I don’t want politicians up the street making these decisions.’”

Mr. Brown has also laid out an economic message that has proved potent in past elections, saying when “people go to the grocery store, they’re paying higher prices because of executive bonuses, and because of stock buybacks.”

“When a CEO makes 300 times what a worker makes, something’s wrong,” Mr. Brown added. “And that’s a big part of this, and you start talking about the economy, it’s always about, ‘Who’s side are you on?’ and voters in Ohio know whose side I’m on.”

What little polling has been done on the race appears to show that Mr. Moreno would perform worse against Mr. Brown than Messrs. LaRose or Dolan. In a matchup between Messrs. Brown and Moreno, the senator enjoys 42 percent support to Mr. Moreno’s 32 percent, according to an Emerson College Nextar, WJW Cleveland survey. Messrs. LaRose and Dolan polled at 36 percent and 38 percent support, respectively.