COLUMBUS, Ohio — President Trump is endorsing “Hillbilly Elegy” author James David Vance in Ohio’s competitive Republican Senate primary, ending months of jockeying in a race where his backing could be pivotal.

In a statement, Mr. Trump described Mr. Vance as “the candidate most qualified and ready to win in November.”

“We cannot play games. It is all about winning!” he wrote.

The decision is a major blow for former state treasurer Josh Mandel, investment banker Michael Gibbons, and former Ohio Republican Party chair Jane Timken, who have been locked in a heated and contentious race for both the nomination and Trump’s backing in a primary that is now less than three weeks away.

On Thursday night, dozens of Republican leaders in Ohio mounted a last-minute effort to urge Mr. Trump not to endorse Mr. Vance following a news report that said Mr. Trump was planning to do just that.

While Mr. Gibbons and Mr. Mandel had been leading in recent polls, officials backing various candidates had long conceded that Mr. Trump’s coveted support would likely push his pick to the front of the pack in a race that has revolved, for a large part, around him.

In addition to trips down to Mar-a-Lago, the candidates and their affiliated super PACs have spent millions trying to paint one another as insufficiently loyal to the former president, who remains deeply popular with the GOP base, despite being impeached twice, and won the state in both 2016 and 2020.

Mr. Vance, in particular, has come under fire for old audio and since-deleted tweets in which he called himself a “never-Trump guy,” called Mr. Trump an idiot, and said he might have to hold his nose and vote for Democrat Hillary Clinton in 2016.

But Mr. Vance has since changed his tune, saying he regrets his past comments. He has closely aligned himself with the former president, making frequent appearances on Fox News and former Trump strategist Stephen Bannon’s podcast, where he has echoed Mr. Trump’s rhetoric on issues including immigration.

In an interview earlier this week, Mr. Vance told talk radio host Hugh Hewitt that he wasn’t sure whether Mr. Trump would end up endorsing in the race, but thought if he did it would come before Mr. Trump returns to the state for a rally on April 23.

“I think that he will. I’m not sure that he will,” said Mr. Vance. “He wants to endorse the person he thinks can beat Tim Ryan. He also wants to endorse a winner.”

The winner of the May 3 primary is likely to face Democratic frontrunner Timothy Ryan in November for the seat being vacated by retiring Senator Portman, a Republican.