The mayor said it was a question of electing "better Democrats" who would "put working people back at the heart of politics." The approach consternated some in Democratic leadership, but the outcome showcased Mamdani's rising influence.
There were several high-profile races across New York on Tuesday.
In New York's 7th Congressional District, ABC News projects that Mamdani-backed state Assemblywoman Claire Valdez will win over Brooklyn Borough President Antonio Reynoso.
In the 10th District, another Mamdani-backed candidate, former city comptroller Brad Lander will beat incumbent Rep. Dan Goldman, ABC News projects.
And in the 13th District, the mayor's candidate, community advocate Darializa Avila Chevalier, will beat incumbent Rep. Adriano Espaillat, ABC News projects. It's one of the biggest upsets in a New York House primary since Alexandria Ocasio-Cortez defeated Joseph Crowley in 2018.
All three victors are expected to win their blue districts, which would also place three Mamdani allies in Congress come January.
In New York's 12th Congressional District, ABC News projects Micah Lasher has won the primary.
He has decades of governmental and political experience and is backed by outgoing Rep. Jerry Nadler, Gov. Kathy Hochul and former Mayor Michael Bloomberg.
In an upstate race to replace retiring New York Republican Rep. Elise Stefanik, ABC News projects businessman Anthony Constantino, who made headlines in 2024 for placing a large "Vote for Trump" sign atop his Mohawk Valley Sticker Mule factory, will be the Republican nominee in the 21st District.
Constantino was supported by President Donald Trump, who released a video in support of his endorsee on Monday. The race highlighted a divide in the Empire State's Republican Party, with the chair of the New York Republican State Committee backing Robert Smullen, a state assemblyman and retired Marine Corps officer.
Constantino will face dairy farmer Blake Gendebien, who ABC News projects will win the Democratic nomination, in November.
And in another race that could determine control of Congress, ABC News projects that former National Security Council Counterterrorism Director Cait Conley will win the Democratic primary to take on Republican Rep. Mike Lawler in the purple 17th District.
(ABC News and The Associated Press contributed to this report.)
----------
* Get Eyewitness News Delivered
* More New York City news
* Send us a news tip
* Download the abc7NY app for breaking news alerts
* Download our connected TV app