In a one-party town, the party boss reigned supreme.

Nowhere was this truer than in New York City, where the shadow of William M. Tweed, known to most everyone as Boss Tweed, loomed over Democratic politics for generations. Slowly, the grip of party leaders waned elsewhere. But in Queens, the machine rolled on — until Tuesday.

If Representative Joseph Crowley’s loss this week to a young insurgent candidate, Alexandria Ocasio-Cortez, rattled the national Democratic Party, it did a great deal more to upend the political order in New York City, where Mr. Crowley was perhaps the last powerful party boss in a city once defined by them.

Under Mr. Crowley, the Queens Democratic machine played local kingmaker, holding sway over judicial races, Surrogate’s Court, even the speakership of the New York City Council. Very little of consequence seemed to occur in Queens politics that Mr. Crowley or his cohorts did not have a hand in.

“It’s the end of an era,” said Councilman Daniel Dromm of Queens, the chairman of the finance committee, an influential position he garnered after aligning himself more closely with Mr. Crowley.

The era of the party boss had long been on the wane across the nation. Gone are the days when James Michael Curley loomed over Boston, the Daleys dominated Chicago and E.H. Crump controlled Memphis.

Old-style party machines have been variously felled by court decisionslimiting political patronage, corruption scandals, the expansion of voting rights, and, more recently, an increase in polarization, political scientists said. Powerful Democratic organizations still exist in big cities, but they have become weaker.

“Political machines have been on the decline almost everywhere for decades, and that process has accelerated,” said Larry J. Sabato, the director of the Center for Politics at the University of Virginia. Those who get involved in politics now are more likely to view it as a cause than a means to a patronage job or a payoff, he said.

“Crowley is a perfect example,” Mr. Sabato added. “He grew up in another time and another way of doing business.”

Mr. Crowley was a product of Queens, New York City’s most stubborn stronghold of party bosses. The Democratic machine there persevered even after the 1986 suicide of its powerful leader, Donald R. Manes, who took his own life amid a corruption scandal. It marched on despite demographic changes that transformed the borough from a bedroom community synonymous with the cantankerous conservatism of Archie Bunker to the teeming home of some of the nation’s most diverse neighborhoods — and an increasingly progressive Democratic base.