Zohran Mamdani was wooing Black pastors at the Rev. Al Sharpton’s Harlem headquarters recently when he mentioned he had been there before, at a Christmas Day event to help people in need.

The pastors, surprised, decided to check up on Mr. Mamdani’s claim. “We had to go look at the tape,” Mr. Sharpton said. “There’s Zohran, serving meals. We didn’t know who he was.”

Mr. Mamdani, 33, is hardly unknown now. His upset win in New York City’s Democratic mayoral primary has electrified Democratic voters across the country, introducing an exciting new name to national politics with a broad coalition of affluent and middle- and working-class voters, Asian, Muslim, Latino and white voters and younger voters of all backgrounds.

It’s the latest example of a Democratic Party in metamorphosis, as a group of maverick progressives and younger people gain power, animating the base and eclipsing some of the party’s longtime kingmakers.

Progressives have a shot at shaping the direction of a party whose leaders have failed to mount a successful opposition to Donald Trump. Before they can offer Democrats a path forward nationally though, they will have to make significant headway with Americans at the very heart of the Democratic coalition: Black voters. Especially older ones.

In districts that were overwhelmingly Black, Mr. Mamdani lost voters to his main opponent, former Gov. Andrew Cuomo, by more than two to one. Progressives seem to particularly struggle with this part of the base. Democrats with big aspirations may want to get very curious about why.