William F. Weld, who once dove merrily into the murky muck of the Charles River to prove his environmental bona fides, has always seemed to relish a sudden plunge into the unknown, even if it might cause him some embarrassment.

In a long, colorful, and peripatetic political career, the boar-hunting, Grateful Dead-loving Republican has tried and failed to become a US senator, ambassador to Mexico, and governor of New York. And he has endorsed both Republicans and Democrats for president.

So when Weld, now a 70-year-old lobbyist, announced Thursday that he was making another political U-turn and launching a quixotic campaign for vice president on the Libertarian ticket, friends and former associates were more amused than surprised.

“It’s a Weld fantasy,’’ Jody Dow, a former longtime Republican National Committee member from Massachusetts, said with a laugh. “He likes to have fun.’’

Maurice Cunningham, a political scientist at the University of Massachusetts Boston, said when he heard the news, “I guess like a lot of Massachusetts folks, I had a giggle at it, because he’s such an interesting character.’’

“It’s quirky, but that’s him,’’ Cunningham said. “That’s the word we always associate with Weld.’’

The scion of a wealthy Long Island family, Weld began his career working for the House impeachment committee investigating President Richard Nixon and later served as US attorney for Massachusetts in the 1980s.

In 1990, he was elected governor of Massachusetts, replacing Michael S. Dukakis and becoming the first Republican to hold the office in 16 years.

Over the next six years, he charted a socially liberal, fiscally conservative course that established one of the most durable brands in Massachusetts politics.

He cut taxes repeatedly, and boasted that he was “to the right of Attila the Hun’’ on crime and wanted to reintroduce prisoners to “the joys of busting rocks.’’ At the same time, he said he was “green as a grape’’ on environmental issues, and supported gay rights and abortion rights.

In 1993, he signed the Education Reform Act, a landmark bill that increased funding for schools in exchange for imposing higher standards.

Two years later, he pushed through legislation that limited welfare benefits and forced more recipients to work. That set the template for the federal welfare reform bill that President Bill Clinton signed a year later.

With wit and charm, he won allies in the prickly, Democrat-controlled Legislature. Senate President William M. Bulger, whom Weld had vilified on the campaign trail, became his song-and-dance partner at the annual St. Patrick’s Day political roast.

Yet Weld was clearly restless on Beacon Hill. In 1996, two years after winning reelection with a stunning 71 percent of the vote, he mounted an unsuccessful bid to unseat Senator John F. Kerry.

A year later, Weld abruptly resigned, in an effort to become ambassador to Mexico.

But his appointment was doomed by Senator Jesse Helms, the conservative North Carolina Republican and chairman of the Senate Foreign Relations Committee, who objected to Weld’s support for medicinal marijuana and needle exchange programs.

After a sojourn as an author of thrillers and historical novels, Weld decamped to Manhattan and became an investment manager.

In 2005, yearning for a return to political life, he launched a long-shot campaign to become governor of New York, and the first two-state governor since Sam Houston led Texas and Tennessee in the 19th century.

Three years after that campaign collapsed, he spurned the GOP and endorsed Barack Obama for president, only to reverse course again four years later and endorse Obama’s Republican opponent, Mitt Romney.

Given those eclectic career choices, Rob Gray, a former Weld spokesman, said he was not surprised that his former boss is now seeking the Libertarian nomination for vice president, on a ticket with Gary Johnson, a former Republican governor of New Mexico.

“Bill is a political free spirit, and he’s at the stage in his career where he can say, ‘Why not?’ ’’ Gray said.

Still, Gray said he was not planning to vote for Weld in November.

“I expect to support the Republican nominee,’’ he said, adding that he doesn’t believe Weld’s campaign is “going to turn into much come the fall.’’

John Moffitt, who was Weld’s campaign manager in 1990, said he, too, plans to shun the Johnson-Weld ticket, even though Weld served as best man at his wedding.

“That’s the way it works sometimes,’’ said Moffitt, who is chairman of the Andover Republican Town Committee. “You join a team, and you stay with that team. I don’t want to do anything to allow Hillary Clinton to become president.’’

Still, Moffitt said he wasn’t angry at his friend for mounting one more campaign, saying it would at least allow Weld to indulge his passion for travel.

“I’m going to tell him to enjoy himself,’’ Moffitt said. “At this point, he has nothing to lose by doing it.’’

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