President Trump endorsed Representative Andy Barr in the Republican Senate primary race in Kentucky on Friday, moving to clear the field by inviting an Elon Musk-backed contender to join his administration.

Nate Morris, the candidate supported by Mr. Musk, said he would leave the campaign, which until Friday had been a hotly contested three-way primary race to replace Senator Mitch McConnell, a longtime pillar of the Republican establishment who is set to retire at the end of the year.

Mr. Morris made his announcement shortly after Mr. Trump said on Truth Social that he had had a “great meeting” with Mr. Morris a day earlier and “asked” him to leave the race. The president said that he planned to appoint Mr. Morris as a U.S. ambassador.

Mr. Morris’s departure narrowed the race in the deep-red state to two contenders: Mr. Barr and Daniel Cameron, a former Kentucky attorney general.

Mr. Trump wrote on social media that Mr. Barr would “fight tirelessly to Grow our Economy” and to “Champion the Interests of our Amazing Farmers and Ranchers.”

Mr. Barr said in a statement that he had been with Mr. Trump “all the way” and would “stand with President Trump 100 percent to deliver for Kentucky and to keep Making America Great Again.”

Scott Jennings, a CNN analyst and Republican strategist in Kentucky, said the president’s move would leave Mr. Cameron with little hope in the primary, predicting that Mr. Barr would win comfortably. “It will be definitive,” Mr. Jennings said.

Mr. Cameron’s campaign did not immediately respond to a request for comment on Mr. Trump’s endorsement of Mr. Barr, which came less than three weeks before the May 19 primary.

Mr. Morris, the wealthy founder of a waste management company and a friend of Vice President JD Vance, has deep ties in Mr. Trump’s orbit. Last year, Mr. Morris announced his candidacy on a podcast hosted by Donald Trump Jr., the president’s son. And Mr. Musk donated $10 million to a group supporting Mr. Morris’s campaign.

The group backing Mr. Morris spent all of that money and then some, financing a nearly $15 million paid media campaign — and it appeared to make little difference. The group did not report any independent expenditures in a month, and it entered April with just $120,000 on hand. Some supporters of Mr. Musk and Mr. Morris acknowledged in recent weeks that the push did not work, according to two people with knowledge of their reaction, who spoke on the condition of anonymity.

Mr. Morris quickly joined Mr. Trump in his endorsement, writing on social media that it was “time for all Kentuckians to rally behind our next Senator, Andy Barr!”