BEDFORD, N.H. — William F. Weld, the former Republican governor of Massachusetts, unleashed a lacerating attack on President Trump Friday, depicting him as a threat to American values and international stability, as he launched an exploratory committee for the 2020 Republican primary.

Weld, who would become the first Republican to confront Trump in the primary, accused the president of praising authoritarian leaders, humiliating allies, tossing around threats of nuclear war, and railing against the rule of law.

“We cannot sit passively as our precious democracy slips quietly into darkness,’’ Weld declared at the Politics & Eggs breakfast in Bedford, N.H., a traditional stop for would-be presidential candidates. “Congress must do its duty and, as citizens, we must do ours.’’

An advocate of free trade, increased immigration, and action to combat climate change, Weld, 73, seems deeply out of step with the direction of the GOP under Trump, who remains highly popular among Republican voters.

But Weld said he wants to at least toughen the president’s path to a second term. Weld pointed out that, of the last nine presidents who ran for reelection, the four who lost had primary challengers.

“That’s a nice statistic,’’ he said.

Not surprisingly, the reception to Weld’s announcement from Republican leaders was less than enthusiastic.

Many pointed out that Weld, a longtime supporter of abortion rights, endorsed Barack Obama in 2008, then declared himself “a libertarian for life’’ and ditched the GOP to become the Libertarian vice presidential nominee in 2016. In the closing days of that race, Weld praised the Democratic nominee, going on TV to declare, “I’m here vouching for Mrs. Clinton.’’

“The Republican Party is a big tent, but someone who endorsed Barack Obama in 2008 and 2012 and supported Hillary Clinton in 2016 as the Libertarian Party’s vice presidential nominee really needs to think about how welcome he is in the Republican Party,’’ said Stephen Stepanek, chairman of the New Hampshire GOP and former state chairman of Trump’s campaign. “I don’t expect his campaign to get very far among Republican primary voters.’’

In Massachusetts, Weld served as the popular Republican governor from 1991 to 1997 with a fiscally conservative, socially moderate agenda that set the template for the four subsequent Republican governors. Here, the party also scorned Weld’s potential candidacy.

“Even Benedict Arnold switched allegiances less often!’’ said Jim Lyons, the chairman of the Massachusetts GOP and a Trump supporter. “We Republicans will put partisanship aside, reach across the aisle to Democrats and Libertarians, and reject Bill Weld.’’

On Friday, Weld defended his decision to rejoin the GOP as pragmatic, saying a primary challenge, not a third-party bid, is the only way to directly take on Trump.

“I want to not dribble around the court, but go right to the hoop,’’ he said, answering questions from some skeptical audience members after his speech in the first primary state. “And if you want to go one-on-one here, it’s got to be as an ‘R.’ ’’

Weld said Republican leaders in Washington “exhibit all the symptoms of Stockholm syndrome, identifying with their captor’’ but Republican voters and independents can still change course.

“As we move toward the 2020 election year, each of us must also strive to remember and uphold the difference between the open heart, open mind, and open handendess of patriotism, versus the hard heart, closed mind, and clenched fist of nativism and nationalism,’’ Weld said.

Tom Rath, a former New Hampshire attorney general and elder statesman in the party, called the speech “vintage Weld,’’ thoughtful and smart.

“He’s somebody that is clearly a person of substance, and he’s running for the right reasons,’’ Rath said. “The issue is, will this party countenance any kind of dissent? And that we don’t know.’’

Weld, a former US attorney who lives in Canton, currently serves on the board of a cannabis investment company and works at Mintz Levin, a major Boston lobbying firm. He is one of several Massachusetts candidates eyeing the White House. Senator Elizabeth Warren has already entered the race, seeking the Democratic nomination.

Governor Charlie Baker, a former Weld Cabinet official who has often criticized Trump, considers Weld “a friend and a mentor’’ but is focused on his own agenda, “not presidential politics,’’ a Baker spokesman said.

Weld said he hopes to encourage other potential Republican candidates such as Governor Larry Hogan of Maryland and former governor John Kasich of Ohio to challenge Trump.

“If I get traction, and cracks appear in the dam of the president’s support, I think you may see a gold rush,’’ Weld said.

Hogan declined to comment, according to an adviser.

John Weaver, a Kasich adviser, responded warmly.

“We have tremendous respect for Governor Weld and consider him a friend and kindred spirit,’’ Weaver said. “Having said that, perhaps we’ll see him in New Hampshire before this is over.’’

Michael Levenson can be reached at mlevenson@globe.com. Follow him on Twitter @mlevenson.