SALT LAKE CITY — Seizing new fuel for his appeal to Donald Trump's critics, former New Mexico Gov. Gary Johnson has joined forces with another former Republican governor to strengthen his Libertarian presidential bid.

William Weld, who served two terms as the Republican governor of Massachusetts in the 1990s, will announce plans Thursday to seek the Libertarian Party's vice presidential nomination, Johnson confirmed in a Wednesday interview with the Associated Press. The pair met privately in Las Vegas over the weekend when Weld agreed to run as Johnson's running mate in the party's upcoming nominating convention and into the general election.

"We got together and shook hands on it," Johnson told the AP in an interview in Salt Lake City, where his underdog presidential campaign is based. "It brings an enormous amount of credibility to what it is I'm doing. I'm unbelievably flattered by this and humbled."

Johnson is casting himself as the best — and perhaps only — alternative to Trump, as the New York billionaire's Republican critics struggle to identify another third-party candidate.

Johnson earned just 1 percent of the national vote during his 2012 presidential run, but reminds reluctant conservatives that he'll likely be the only third-party candidate on the ballot in all 50 states this fall.

Weld, a well-respected former governor in the Northeast, offers Johnson some credibility and badly needed fundraising prowess. The 70-year-old will announce his vice presidential bid in New York on Thursday, Johnson said.

"He could be a huge influence when it comes to fundraising. Huge," said Johnson, whose campaign had only $35,000 in the bank at the end of March. "That was something that he in fact volunteered — that he enjoys doing it."

Weld was an active fundraiser for 2012 Republican presidential nominee Mitt Romney. Like Johnson, Weld has a moderate view on social issues. He favors abortion rights and same-sex marriage. Years after leaving the governor's office in Massachusetts, Weld launched an unsuccessful bid for New York governor as a Republican and a Libertarian.