If you were going to guess Al Giordano's political preferences based on his biography, you'd be certain he was a Bernie Sanders supporter.

Giordano, a 56-year-old journalist and organizer, began his career working against nuclear power plants in New England, then worked with Abby Hoffman through the 1980s. He wrote for The Nation in the 1990s, before leaving the U.S. to report on the Zapatistas in Mexico and on the ravages of the drug war. In 2008, Giordano was a vocal and animated supporter of Barack Obama — and an impassioned critic of Hillary Clinton. It wouldn't be crazy to suspect he has a "Feel the Bern" tattoo.

So it's quite a surprise to discover that Giordano backs Hillary Clinton — is not an admirer of Bernie Sanders at all.

Giordano has become so disenchanted with Sanders that he's contemplating a challenge to the democratic socialist's Vermont Senate seat in 2018. If Sanders doesn't enthusiastically support Clinton as the nominee by the convention, Giordano says, he'll establish residency in Vermont and start working on a Senate campaign.

This seems like a dizzying shift. But Giordano explains that he's not so much committed to Clinton as a candidate as he's committed to "the Obama coalition." "The future of progressive politics is in the multi-racial coalition," Giordano told me by Skype from his home in Mexico City.

Giordano thinks Sanders has disrupted that critical progressive coalition. The Vermont senator "has a blind spot on racial justice issues," Giordano argues. He is "exploiting racial and gender divisions... in a way that harms the movement." For instance, Sanders' comments about the illegitimacy of the primary process, and dismissal of Clinton's victories in Southern states, which were fueled by black voters, have "poisoned the well," Giordano says, and made unity against the Republicans difficult. That divisiveness, he believes, "also harms the people of Vermont. He's going to be a pariah in Congress. He's not going to be able to get anything done for Vermont."

Giordano's impetus for challenging Sanders in 2018, then, is explicitly about party coalitions, cohesion, and pragmatic efforts at change. He is threatening to oppose Sanders because Sanders is, in his view, threatening the unity of the Democratic Party coalition.

This is a very unusual basis for a challenge to a sitting senator in the modern era, when anti-incumbent fervor tends to be ideologically based. The Tea Party generally takes on Republican incumbents because they are too centrist, or insufficiently ideologically pure. No one challenges a sitting senator because they are not sufficiently committed to party unity.

But according to Marquette University political scientist Julia Azari, coalition challenges used to be quite common. "In the pre-primary period, in the 19th century party period, this was exactly how party politics worked," Azari told me by phone. "We didn't have primaries, but the main glue holding the party together was supporting the presidential nominee, and if you didn't do that, you were going to be in trouble." In some cases, "the party might support someone running for Congress in your place." In that sense, she concluded, Giordano's challenge is "kind of a throwback."

So would this throwback be successful? Can Giordano win Sanders' Senate seat? Christina Amestoy, the Vermont Democratic Party spokesperson, was skeptical. "Sen. Sanders is still incredibly popular in the state," she told me. "He's been so popular in the state, people are really excited to see his message resonating." The Democrats' 2016 primary divisions "can easily be healed," she said.

And indeed, if Sanders endorses Clinton enthusiastically by the convention and works to defeat Trump, Giordano says he'll drop his plans for a challenge. But if Sanders continues to try to delegitimize Clinton during and after the convention, Giordano promises to challenge him — and he believes he can win. There has been a lot of enthusiasm from prospective donors, he says. And he believes he also has organizing skills that Sanders lacks.

Both Sanders and Giordano are very left-wing candidates; on most substantive issues, they're quite close ideologically. Their differences are mostly strategic and practical. Is the Democratic Party a barrier to real change? Or are left-wing goals best achieved via organizing, and building coalitions, within the Democratic Party? Sanders' successes and failures have forced these questions to the forefront for many on the left. They'll come up again and again in the years ahead — perhaps even in 2018 in Vermont.