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“I do think there is an emerging electorate of blacks and whites who identify as progressive Democrats,” Mr. Wells, who endorsed Mr. Grosso, said Wednesday. He said the nascent bloc of voters are distinguished “more by age than race, and Grosso tapped into that.”

Mr. Mendelson’s victory in the special election to serve as chairman through 2014 — completing the term left by Kwame R. Brown, who resigned and pleaded guilty to bank fraud in June — means he will officially vacate his at-large seat.

City voters will select a person to fill the seat in a special election this spring, and on Tuesday night Mr. Brown said he has not decided whether he will run. Instead, he kept an upbeat posture while thanking his campaign aides individually, shaking their hands and imploring them to hit up the food tables.

“Just take a whole pie,” he said, “don’t take a slice home with you.”

Mr. Mendelson showed up to share a few words with Mr. Brown. About an hour later, the chairman visited Mr. Grosso’s party for the first of many handshakes and photo opportunities.

Mr. Grosso, when asked if he thought his victory marked a pivotal moment in D.C. politics, paused for a second.

“Heck yeah!” he eventually replied. “Don’t you?”

Note: An earlier version of this story included an incorrect date for the last time a challenger unseated an elected D.C. legislator. The error has been corrected.