Socialist firebrands Mayor Zohran Mandani and Vermont Sen. Bernie Sanders warned of a new radical “movement” sweeping the nation in a rally Thursday night to boost a slate of lefty Big Apple House hopefuls.

The progressive bullhorns juiced the packed “Our Year, Our Team” crowd at Kings Theatre in Flatbush to support a slate of Mamdani-backed Democratic Socialists of America congressional candidates for the pivotal June 23 Democratic primaries.

The candidates — two of whom are pitted against establishment Dems — include former New York City Comptroller Brad Lander, running in the 10th House District, Democratic socialist state Assembly Member Claire Valdez for the 7th House District, and Darializa Avila Chevalier for the 13th House District.

“The Democratic Party must change. The party of the past will not be what leads us into the future, for we need a democratic party with a backbone,” Mamdani told the raucous crowd.

The mayor, wearing a Knicks Jersey under his suit mere hours after a ticker-tape parade through the Canyon of Heroes celebrated the team’s NBA championship, urged New Yorkers to “show up” for the candidates as they showed up for him in his historic November win.

“Our work has never been about any one person or any one office or any one election. Our work has been about a movement. A movement to turn the page on the politics of big money and small ideas,” Mamdani said.

“A movement to usher in a new era for our city. A movement to make government work for every single New Yorker, not just the wealthy and the well-connected.”

Sanders, a Flatbush native, pointed to progressive wins sweeping the nation and said it was crucial to carry that momentum across the Big Apple.

“In the last eight months, progressive, democratic socialist candidates all over this country have been winning major victories,” the Vermont firebrand shouted.

“Why are progressives and socialist candidates winning elections all across this country? The working class of America understands that our current economic system is rigged,” Sanders added.

“That it is designed to benefit the wealthy and the powerful, and while the very richest people in this country become richer, it leaves working families behind, struggling to put food on the table.”

Valdez, Chevalier and Lander all spoke at the get-out-the-vote rally, which was emceed by comedian Ilana Glazer and featured a performance by Sarah Bareilles.

Valdez, who is running to represent part of Brooklyn and Queens in the June 23 primaries, was backed by Mamdani over Brooklyn Borough President Antonio Reynoso, who was retiring Rep. Nydia Velázquez’s pick to fill her seat.

Valdez charged “solidarity does not stop at our borders.”

“The war machine and the billionaire class are not separate people; they are the same. But they are not as powerful as they think because when working people come together, they can move mountains,” she slammed.

Lander, who is challenging Rep. Dan Goldman, also blasted establishment Democrats and billionaires.

“We’ve had it with corporate democrats who don’t know what time it is,” he raged.

“This is not the time for strongly worded letters, or high-dollar fundraisers, or elections bought by billionaires, or crypto bros, or AI oligarchs. It’s time for politics of solidarity.”