President Trump endorsed Representative Mike Collins on Sunday in the Republican Senate primary runoff in Georgia, choosing a loyalist and immigration hard-liner over a former football coach who had angled for his support.

By backing Mr. Collins over Derek Dooley, a former football coach at the University of Tennessee, Mr. Trump gives the congressman a major lift as he seeks to win the nomination to take on Senator Jon Ossoff, a Democrat, in one of the nation’s most competitive midterm battlegrounds.

“Mike Collins is a true Friend, Fighter, and WARRIOR, who has been with us from the very beginning, and has my Complete and Total Endorsement to be your next United States Senator,” Mr. Trump wrote on Truth Social early Sunday morning, two days before the runoff.

Mr. Collins led Mr. Dooley by about 10 percentage points in an initial round of primary voting in mid-May that included another Trump acolyte, Representative Buddy Carter. Opinion polls have shown Mr. Collins leading Mr. Dooley in the head-to-head matchup.

But Mr. Dooley has the backing of Gov. Brian Kemp of Georgia, a popular Republican, and Republican strategists in the state had said they expected the runoff to be close. Mr. Trump’s endorsement, which has proved immensely powerful in Republican primary after Republican primary in recent weeks, could scramble that calculus.

Mr. Collins, a trucking executive with a history of incendiary social media posts, sponsored the first bill Mr. Trump signed after returning to the presidency. The congressman’s campaign has also brought on some of the president’s political advisers, including Mr. Trump’s pollster, Tony Fabrizio, and Tim Saler, a data analyst for his 2024 campaign.

Mr. Dooley had worked to appeal to the president by visiting the White House for a lengthy meeting last summer and using a campaign slogan, “Georgia First,” that echoed the president’s “America First” message. Mr. Trump has a history of warming to sports figures who enter politics.

But Mr. Dooley’s chief political patron, Governor Kemp, has had a tumultuous relationship with the president after refusing to join Mr. Trump’s effort to reverse the 2020 presidential election. As recently as 2024, Mr. Trump publicly referred to Mr. Kemp as a “bad guy,” though they smoothed out their public relationship by the election that year. Mr. Kemp has invested heavily in the race, joining Mr. Dooley at dozens of campaign events across the state.

In his post, Mr. Trump wrote that Mr. Dooley “seems like a nice person.” He then returned to his false claims that he carried Georgia in the 2020 election, writing disapprovingly that Mr. Dooley “said that I lost Georgia in 2020.”