One of Ben Carson’s top fundraisers told the Republican presidential candidate on Wednesday that he is quitting the campaign after he was unable to orchestrate a shake-up of its senior staff.

Bill Millis, an heir to a High Point, N.C., sock-manufacturing fortune, told the retired neurosurgeon in an email Wednesday morning that he is leaving the campaign’s three-man board, Mr. Millis said in an interview. He still believes Mr. Carson would make the best president, he said, but is no longer confident the staff values his input.

“I disagree with the campaign, but I’m hoping and praying that the concerns I have are wrong,” Mr. Millis said. “I’m one, and they are the masses. And they decided to move forward with the campaign as is.”

Mr. Millis’s departure comes after several weeks in which Mr. Carson has struggled to articulate his foreign-policy vision. At the same time, Mr. Carson’s top staffers have pushed out Terry Giles, the candidate’s friend of 20 years who had been responsible for hiring the campaign’s leadership.