Will Libertarian Bill Weld stay in his lane or return to his Republican roots?

William Weld, then the Libertarian vice presidential candidate, takes the stage to speak at a 2016 rally in New York. AFP/Getty Images

Can a former Republican governor of Massachusetts who ran for the Senate, tried to become ambassador to Mexico, ran for governor of New York, endorsed Barack Obama in 2008, and was the Libertarian Party nominee for vice president in 2016 be taken seriously as a presidential candidate in 2020?

William F. Weld is about to find out.

In the latest twist in a long and winding political career, Weld is seriously considering challenging President Trump. The big question seems to be whether he will run as a Libertarian or as an insurgent in the Republican primary, an idea that is already infuriating Trump supporters and intriguing establishment Republicans eager to see a mainstream alternative to the president.

“You cannot overestimate the depths of Trump’s support among hardline Republicans,” said Tom Rath, a former New Hampshire attorney general and elder statesman in the party. “There is this group in the middle, however, that in the moment is unrepresented by any party and is anxious to find a champion.”

Some say the best hope for Weld would be to confront Trump directly in the Republican primary, exploiting his proximity to New Hampshire and his fiscally conservative, socially tolerant views to appeal to moderate Republicans and independents. Even a strong second place showing could hurt Trump, Weld allies say, much the way Pat Buchanan captured 37 percent of the vote against President George H.W. Bush in the 1992 New Hampshire primary and presaged Bush’s eventual loss to Bill Clinton.

“If the goal is defeating Trump, the best option is for him to run as a Republican because primary challenges have historically weakened sitting presidents,” said Rob Gray, Weld’s former press secretary and a Republican strategist. “Primary challenges get more media coverage and have more political juice so it puts an incumbent in the position of fighting a two-front war, against candidates of a different party and against someone in their own party.”

But running as a Republican would force Weld to break an ironclad promise he made in 2016, when he narrowly won the vice presidential nomination at the Libertarian convention by declaring, “If you hear nothing else from me, hear this: I pledge to you that I will stay with the Libertarian Party for life.” Some Republicans regard him as a traitor for ditching the GOP and for comparing Trump’s immigration policies to Kristallnacht, the 1938 Nazi rampage that destroyed Jewish homes, businesses, and synagogues.

“Let’s face it: He turned his back on the Republican Party in 2016 and we, quite frankly, don’t want him back,” said Stephen Stepanek, the newly elected chairman of the New Hampshire Republican Party who was cochairman of Trump’s New Hampshire campaign in 2016. “I don’t see any room in the Republican Party for someone who has turned their back openly and publicly and gone with another party to come back two years later and say, ‘I changed my mind.’ ”

The other option for Weld, running as a Libertarian, would allow him to keep his promise to remain in the party. He has spent the last two years wooing Libertarian activists at conventions across the country, most recently at an event for liberty-minded students in Washington, D.C., earlier this month.

Weld greeted supporters in 2016 in Concord, N.H., during a rally for the Libertarian presidential ticket.Mark Lorenz for the Boston Globe

But as a former Republican, onetime federal prosecutor, and member of the Council on Foreign Relations, he is still viewed skeptically by many in the Libertarian base, and might face a tough time winning the presidential nomination at the party’s national convention in 2020. Representative Justin Amash, a Michigan Republican popular with many Libertarians, has floated himself as a possible candidate and taken some not-so-veiled jabs at Weld, calling him “squishy.”

Running as a Libertarian might also allow Trump to dismiss Weld as a third-party gadfly, denying the former governor the opportunity he so desires to tangle directly with the president.

“Nobody is going to invite Trump to debate the Libertarian candidate in New Hampshire,” Gray said. “Whereas, it is likely someone is going to invite the Republican primary candidate to debate him in New Hampshire. Trump can avoid it, but there’s a political risk to that.”

Still, many Libertarians, pointing to Weld’s repeated statements of support for the party, say they expect Weld to remain a Libertarian and be active in 2020 as a candidate himself or as a supporter of the eventual nominee.

“I would absolutely be upset if he went back to the Republicans — most, if not all, Libertarians would be,” said Jeff Lyons, chairman of the Massachusetts Libertarian Party. “He’s definitely made a lot of comments up to a week and a half ago, saying ‘I’m going to be involved with Libertarians in 2020.’ ”

Weld is declining to detail his thinking for now, and is expected to reveal more of his plans when he speaks on Feb. 15 at Politics & Eggs, a traditional stop for would-be presidential candidates in New Hampshire. He has flatly denied a WCVB-TV report that he has taken a leave from his law firm, Mintz Levin, and that an announcement about a presidential campaign could be imminent.

“No, not correct, either item,” he wrote in an e-mail to the Globe Wednesday night.

Stephen Tocco, a Weld friend and chairman of ML Strategies, Mintz’s lobbying arm, also batted down the report, writing in an e-mail, “He did not take a leave and has not made a decision.”

Weld, 73, would make an unlikely Republican candidate in the Trump era.

But as a longtime supporter of abortion rights and gay rights, he has a history of being proudly out of step with the GOP base, beginning in 1992, when he was booed at the Republican National Convention. Some say he can expect a similarly frigid reception in 2020, if he were to challenge a president who remains popular with the party faithful.

Weld, then a Republican, endorsed Democrat Barack Obama for president at the Obama campaign office in Salem, N.H., in 2008. Cheryl Senter/Associated Press

“There is just not a lane for Bill Weld in the Republican national process and, if he wants to be effective voicing his displeasure with the current administration, he’s best served running as a Libertarian,” said Ryan Williams, a former spokesman for the New Hampshire Republican Party. “The Bill Weld brand of Republican is not as prevalent as it once was, let’s just say that. This is Trump’s party, without question.”

If he runs, Weld — who has dabbled in novel writing and joined the board of Acreage Holdings, a cannabis investment firm — may need to persuade voters that he is truly committed to a campaign, and not merely indulging a political flight of fancy.

He also could face competition for the anti-Trump vote if other Republicans mentioned as potential candidates such as Governor Larry Hogan of Maryland or former governor John Kasich of Ohio were to enter the Republican primary.

Rath said Weld must show voters he is credible by honing a sharp message, building a strong organization, and making repeated visits to New Hampshire.

“He has to be taken seriously and this can’t just be a whimsical thing, and he knows that,” Rath said. “He has to be more than a protest vehicle to gain traction because this vote, to New Hampshire people, is one of their most important political possessions. They don’t give it away in a trivial fashion.”