Mr. Flake, a Mormon known more for his decency than his independent streak, said he had no regrets.

In an interview here, he ticked off some of his earliest criticisms of the president — from the days when Mr. Trump peddled the false theory that President Barack Obama was born in Kenya, to the time Mr. Trump referred to Mexican immigrants as “rapists,” to his call for a complete ban on travel to the United States by Muslims — before looking up and stopping himself.

“In which of those instances,” the senator asked, “should I not have spoken out? At what point should you not stand up and say, ‘This is not right; this is not conservative; this is not where Republicans ought to be?’”