William Weld, the vice-presidential nominee for the Libertarian Party, said Wednesday that he believed Hillary Clinton had done nothing “criminal” in using personal email as secretary of state.

Mr. Weld spoke during a wide-ranging interview alongside Gary Johnson, a former governor of New Mexico and the Libertarian Party’s presidential nominee. Both secured the Libertarian nomination Sunday at the party’s contentious convention in Orlando, Fla., over the weekend.

The remarks from Mr. Weld, who served as the governor of Massachusetts as a Republican in the 1990s, are deeply at odds with criticisms from Republicans over Mrs. Clinton’s tenure at the State Department and the scandal involving her use of a private server for all work emails.

His comments could also fuel further skepticism among some Libertarian voters who view Mr. Weld’s right-of-center years as governor with suspicion. They could also inflame criticism that he is a de facto stalking horse for Mrs. Clinton, the Democratic front-runner for the nomination.