At the age of seventy, Bill Weld, the former Republican governor of Massachusetts, has become the Vice-Presidential nominee for a party to which he’s never belonged. Photograph by Stephen Chernin / AP

Last weekend, when Bob Durand, a longtime environmental lobbyist, arrived at an old buddy’s private camping grounds in the Adirondacks, his host was absent. William Weld, the former Republican governor of Massachusetts, had invited Durand and several other friends to come out for a few days of fishing. But as Durand and the other guests were casting their lines into the lake on the property, Weld was in Orlando, Florida, becoming the Vice-Presidential nominee of the Libertarian Party.

In Massachusetts’s political lore, Durand will be forever linked to Weld. In August, 1996, the two of them dove together into the Charles River, in Boston, to celebrate the passage of the Rivers Protection Act, which placed restrictions on the use of land alongside some nine thousand miles of rivers across the state. Durand, a Democratic state senator at the time, had authored the bill, and Weld, then the governor, had signed it. “He’s such a quirky guy in many ways,” Durand told me this week. He had not seen his friend’s Libertarian conversion coming, but he was not greatly surprised by it.

Weld’s lightheartedness always helped explain his popularity in Massachusetts. He offset the Boston Brahmin élitism he was born into with his unabashed fondness for the Grateful Dead and “amber-colored liquids,” as he once infamously put it. But underneath this joviality is a shrewd and calculating politician. His dive into the Charles with Durand, for example, drew public attention to a bipartisan initiative during Weld’s campaign to unseat John Kerry, then a U.S. senator. After seeing news coverage of the event, Kerry sent Durand a letter. “Stay out of the river with Governor Weld,” Kerry wrote. “You’re costing me votes.”

During his long career, Weld has often projected a sense that he acts on principle, without regard for the consequences. In the early nineteen-seventies, as a counsel with the House Judiciary Committee, he worked on the impeachment of his own party’s President. A few years later, as a U.S. Attorney, he tried to take down the mayor of Boston. In 1988, he resigned from a position in the Justice Department in protest against Ed Meese, who was then the Attorney General. At the 1996 Republican National Convention, he fought a losing battle to remove pro-life language from the Party platform. In 2008, he endorsed Barack Obama for President. And now, at age seventy, he’s joined the Presidential ticket of a party to which he’s never belonged.

Weld is hardly a perfect fit for the Libertarian Party. (In fact, he barely won the Party’s Vice-Presidential nomination, garnering just 50.5 per cent of delegate votes on the second ballot.) His brand of New England Republicanism, now nearly defunct, fully accepts that government has a role to play in the pursuit of the common good. And whereas the Libertarian Party has come to embrace criminal-justice reform and drug legalization, Weld, the hard-nosed former prosecutor, once called himself Attila the Hun on crime issues, and as governor pushed for mandatory minimum sentencing for drug crimes. Other sins, from the Libertarian perspective, can be attributed to the unavoidable realities of Massachusetts politics. As governor, Weld oversaw a growing state budget and supported a ban on assault weapons. Now, as he teams up with his fellow former Republican governor Gary Johnson, who is at the top of the Libertarian ticket, Weld will undoubtedly display another aspect of his character—the ability to adapt to a new reality. He did this in 2006, when he ran for governor of New York, and backed away from his prior support for same-sex marriage in an effort to woo the state’s conservatives. But it should surprise no one if Weld keeps his habit of bucking his own party.

“He will twist and bend to benefit whatever his goal at the moment is,” Arline Isaacson, a veteran Massachusetts lobbyist who tussled with Weld on gay rights, told me. “When it came to L.G.B.T. rights, first he was bad, then he was good, then he was bad, then he was good.”

After his failed 2006 campaign in New York, Weld returned to Massachusetts, where he spent his time lobbying to bring a casino to the Boston area, stoking political rumors about himself, supporting Mitt Romney’s efforts to become President, and helping Charlie Baker, his former protégé, follow his footsteps into the Massachusetts governorship. All the while, he watched as the Republican Party lurched further and further away from the Party he knew, culminating in its takeover by Donald Trump. This might explain why he has jumped into the race. In his first week back in the national spotlight, Weld twice compared Trump’s plan to deport undocumented immigrants to Nazism, first evoking Kristallnacht in a Times interview and later telling MSNBC that Trump’s plan “would remind me of Anne Frank hiding in the attic.”

From now until November, Weld will travel the country, preaching his brand of political propriety. And he’s already won the Libertarian Party at least one new vote. “I was a Hillary Clinton supporter, but now that I have a friend in the race I’ll give him a vote,” Durand told me. “I wish he wasn’t running—I wish he had been there last week. We caught some nice brook trout.”