Senator Lindsey Graham won the Republican primary in South Carolina, according to The Associated Press, after staving off a challenge on Tuesday in a crowded election that had threatened to force him into a runoff.

Mr. Graham, a four-term incumbent, defeated five other candidates, including Mark Lynch, a Greenville businessman who put millions of dollars of his own money into the campaign. South Carolina is a solidly Republican stronghold, and Mr. Graham will be favored to win re-election in November against the Democratic nominee, Dr. Annie Andrews, a pediatrician.

Mr. Graham and his allies ramped up spending in the last weeks of the campaign, as it appeared increasingly likely that he would be forced into a runoff. Since January 2025, Mr. Graham’s campaign and groups supporting him have spent more than $18 million in political ads, according to AdImpact, a media tracking firm.

Earlier this year, Senator John Cornyn of Texas was forced into a Republican runoff that he later lost to Ken Paxton. But in that race, it was Mr. Paxton who had President Trump’s endorsement. In South Carolina, Mr. Trump endorsed Mr. Graham.

In April, Mr. Trump referred to Mr. Lynch on social media as a “LUNATIC” aligned with Representative Thomas Massie of Kentucky, a frequent Trump skeptic whom Mr. Trump helped oust in a Republican primary last month. Mr. Lynch portrayed Mr. Graham as insufficiently MAGA and criticized the war in Iran, which Mr. Graham has defended as necessary to defend Israel’s security.

Mr. Graham is the rare Republican who was able to get into Mr. Trump’s good graces despite having strongly criticized him in the 2016 presidential election. But the senator remains unpopular with many members of Mr. Trump’s MAGA base, who sometimes boo him when he shows up at the president’s rallies.

Matt Zdun contributed reporting.