The duo cross-endorsed one another during New York City's ranked-choice mayoral primary.
Mamdani's endorsement of Lander marked the mayor-elect's most high-profile incursion into national politics since his stunning victory. It comes as other progressives are hoping to capitalize on that momentum with a spate of challenges to moderate Democrats in 2026. Mamdani recently declined to endorse another close ally who was exploring a run against House Minority Leader Hakeem Jeffries.
"Brad's unwavering principles, deep knowledge, and sincere empathy are what make him a true leader," Mamdani said. "He has been a trusted ally and partner of mine and I'm proud to support him as I know he'll continue delivering for those who need government to show up for them the most."
Lander's term as city comptroller ends on Dec. 31 after he decided not to seek reelection.
Now, Lander is looking to represent New York's 10th District, which serves parts of Brooklyn and Manhattan.
Along with Mamdani's endorsement, Lander also scored support from Sen. Bernie Sanders, Sen. Elizabeth Warren and Public Advocate Jumaane Williams. Lander was also endorsed by New York State Sen. Andrew Gounardes and New York State Assemblymember Robert Carroll.
On Tuesday night, Lander was also endorsed by the New York Working Families Party.
Lander touted his decades of public service as a housing organizer, City Councilmember and Comptroller. In his campaign announcement video, he vowed to be a stronger voice to oppose President Trump and included viral footage of his arrest at 26 Federal Plaza.
Early this year, Lander drew national attention when he along with a group of state lawmakers were arrested while attempting to observe the conditions and treatment of immigrants at court in lower Manhattan.
Lander opted to take his case to trial and pleaded not guilty on Friday to low-level misdemeanor charges of trespassing and blocking federal operations.
"While our immigrant neighbors are being demonized and attacked, we can put our bodies on the line to protect them," Lander referenced his arrest and pro-immigrant advocacy in his campaign launch video, which likens his attitude to Fred Rogers in "Mister Rogers' Neighborhood."
"Democracy is just neighbors working together to make our lives in common better," Lander emphasized.
In a statement released Wednesday, the Lander campaign also said he "exposed Donald Trump's theft of $80 million in FEMA funds and forced the City into court to get it back ... saved taxpayers over $2.5 billion by rooting out fraud and waste and investing the City's pension funds wisely ... divested from fossil fuels, and invested in building or saving 50,000 affordable homes."
The NY-10 congressional seat is currently held by Rep. Dan Goldman, also a Democrat.
On Tuesday evening, Rep. Goldman said he is "laser-focused" on the Trump administration and that he has "no ill will towards" Lander.
"Dan is focused on stopping the Trump administration from what they're doing to immigrant families in his district right now. He's proud of his progressive record in Congress and will deal with Brad and other challengers in the new year," a spokesperson for Rep. Goldman said.
Goldman has also taken on the Trump administration and protested ICE raids and may have the upper hand in the race.
"It's very, very difficult, just just by a statistical faction, to beat an incumbent Congress person," said political analyst Hank Sheinkopf.
Sheinkopf says this congressional race reflects a broader theme of the Democratic party trying to figure out where it stands.
"This is an attempt, obviously, by the left, to remove that center left and push the party all the way over to its extremes," Sheinkopf said. "If it's successful, well, it's the end of the center of the Democratic Party. Why? Because if it can happen in New York, it can happen any place else."
And while Lander is texting supporters hoping to raise money, Goldman is an heir to the Levi Strauss fortune and used $5 million of his own money in his last campaign.
(The Associated Press contributed to this report.)
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