He won by making good on promises. One example: A few years after Mr. Sanders left Burlington’s city hall, the mayor’s office was occupied by a Republican, who cut back the budget for snowplowing, then ran afoul of a heavy winter snowfall. In the next election, remembers the University of Vermont sociologist Thomas Streeter, a bumper sticker read: “At least the hippies plowed the streets.”

That is the secret of Mr. Sanders’s style of politics, once called “municipal socialism.” It’s the ’60s reformist impulse without countercultural baggage. It’s the kind of square that needed time to become hip. It’s a moralistic politics that takes seriously the democratic proposition that elected officials must deliver results.

“The fact that he wins elections says there’s an alternative to Clinton-style politics,” says Mr. Streeter, referring to the reliance in the 1990s on conservative budget-balancing. It was precisely that sort of alternative Lee Webb and others promoted in the 1970s, with the Vietnam War over and the former New Left at sea.

At the same time, to stake out what the writer and activist Michael Harrington called “the left wing of the possible,” many ’60s student veterans and others went to work in inclusive “progressive” politics, eventually to triumph in many cities, including Bill de Blasio in New York.

Not that the call of electability always arouses the left. The onerous work of endless meetings, self-sacrificing coalitions and power struggles in local Democratic parties turns off many single-issue activists of the post-’60s. Because deliverable results are so hard to come by, progressives of various ages have often gone for electoral politics of the proudly, defiantly independent sort.

In 1980, the environmentalist Barry Commoner ran a campaign for president as the standard-bearer of a rudimentary Citizens Party. He won fewer than 250,000 votes. The interior of the Democratic Party itself was much more fertile territory for the right kind of candidate. In 1984, Jesse L. Jackson won roughly three million primary votes; in 1988 he won more than double that number.

After the disappointments of the Clinton presidency — spiking inequality even as median income grew — Ralph Nader’s supporters thought their time as outsiders had come around. Years of effort in mainstream party operations and primaries were too compromised for them. (Mr. Sanders solved this problem in Vermont by staying outside the Democratic Party but creating an alternative apparatus that proved it could govern.) So Mr. Nader ran as a Green against Al Gore, and the rest is history. Since 2000, to put it mildly, third-party politics has not been popular on the left.