Representative Scott Rigell of Virginia says he plans to vote for the Libertarian Party’s presidential ticket, becoming the first member of Congress to express support for Gary Johnson’s third-party campaign.

In an interview on Friday, Mr. Rigell, a Republican, said he had settled on Mr. Johnson, the former governor of New Mexico, as the best option available.

“I’ve always said I will not vote for Donald Trump and I will not vote for Hillary Clinton,” Mr. Rigell said. “I’m going to vote for the Libertarian candidate.”