Lifelong Republican Doug Elmets, a former Reagan administration speechwriter and domestic adviser, said Monday he sees Mr. Trump as a “petulant reality TV star.”

“I never anticipated supporting a Democrat, but Hillary Clinton is certainly the most prepared candidate probably in modern times to be president,” Mr. Elmets said in a video interview with The Sacramento Bee. “And given the alternative of a petulant reality TV star candidate in Donald Trump, the decision was very easy.”

Mr. Elmets has made no secret of his opposition to Mr. Trump — he denounced the billionaire businessman in a May 31 interview with CNN — but unlike other prominent #NeverTrumpers, he has taken his antagonism a step further by championing Mrs. Clinton.

“I feel like I am using this opportunity to do what is best for this country,” he said.