WASHINGTON — Joe Walsh, the conservative radio show host and former Tea Party congressman from Illinois who was running a presidential campaign challenging President Trump’s fitness for office, announced on Friday that he was ending his bid for the Republican nomination.

That leaves Mr. Trump with only one Republican challenger, former Gov. William F. Weld of Massachusetts.

“I am ending my candidacy for president of the United States,” Mr. Walsh said in an interview on CNN Friday morning. “I got into this because I thought it was really important that there was a Republican — a Republican — out there every day calling out this president for how unfit he is.”