For years, the conventional wisdom in New York among strategists and candidates alike has been that in any Democratic primary, the road to victory runs through Black communities.

Then came Zohran Mamdani.

In the race that culminated on Tuesday, Mr. Mamdani forged a new multiracial political coalition to become the likely Democratic nominee for mayor and topple Andrew M. Cuomo, the former governor, who had far more name recognition, financial firepower — and political baggage.

And Mr. Mamdani did so even as he lost many of New York City’s most solidly Black neighborhoods. A New York Times analysis of the results shows that Mr. Cuomo dominated in precincts where at least 70 percent of residents are Black, more than doubling Mr. Mamdani’s support, 59 percent to 26 percent.

The result is a break not just from the parochial politics of New York — Black voters helped deliver the mayoralty to both Eric Adams and his predecessor, Bill de Blasio — but from the nation as a whole. Black voters have served as the Democratic Party’s most important voting bloc this century, elevating Barack Obama, Hillary Clinton and Joseph R. Biden Jr. as the party’s last three presidential nominees, oftentimes sanding down the most exuberant instincts of the left.

Most famously, Representative James Clyburn of South Carolina rescued Mr. Biden’s flagging 2020 effort by rallying Black voters before his state’s primary in a bid to thwart Senator Bernie Sanders, though Mr. Clyburn’s backing did not appear to help Mr. Cuomo in this race’s closing stretch.

In a city whose politics have been defined by race-based math, Mr. Mamdani’s success as a democratic socialist upended these traditional calculations and birthed a new and unconventional coalition. It also highlighted tensions between older and more moderate Black voters and the party’s most strident progressive wing, typically anchored by wealthier white voters.