Representative Alexandria Ocasio-Cortez, New York City’s most prominent progressive leader, endorsed Assemblyman Zohran Mamdani for mayor on Thursday, throwing her clout behind an upstart socialist who has galvanized young voters.

The endorsement, shared in an interview with The New York Times, represented perhaps the most significant attempt yet to push fractured progressives toward a single strategy to block former Gov. Andrew M. Cuomo’s path back to power.

“Assemblymember Mamdani has demonstrated a real ability on the ground to put together a coalition of working-class New Yorkers that is strongest to lead the pack,” Ms. Ocasio-Cortez said. “In the final stretch of the race, we need to get very real about that.”

Time is of the essence. With just weeks left before the June 24 Democratic primary, Mr. Cuomo has maintained a steady polling lead, stirring memories of 2021, when progressives did not coalesce and lost the mayoralty to a moderate.

In elevating the assemblyman, Ms. Ocasio-Cortez, 35, chose the candidate closest to her own mold. Mr. Mamdani, 33, is a leftist representing Queens, a millennial and a person of color who entered the race as a long shot only to jump to second place by igniting grass-roots support with memorable policy proposals and viral videos.

His platform echoes the congresswoman’s and has inspired pushback from more moderate Democrats. He wants to make city buses free, freeze rent increases on rent-stabilized apartments and open city-run grocery stores — all funded by a $10 billion tax hike on businesses and the superrich. He has also been a sharp critic of Israel.