If left-leaning voters in New York City needed further spiritual encouragement to support Zohran Mamdani’s bid to become mayor, they got that and more on Monday from Senator Elizabeth Warren of Massachusetts.

Ms. Warren was in full-scale attack mode on Mr. Mamdani’s behalf. In an opinion piece published Monday in Rolling Stone, she wrote that two of his opponents, Mayor Eric Adams and former Gov. Andrew M. Cuomo, were “deeply flawed candidates”; she later defended Mr. Mamdani’s calls to raise taxes during a combative interview on CNBC.

“Oh dear, are you worried that billionaires are going to go hungry?” she said.

And then she joined him on the campaign trail to highlight his proposal to enact free universal child care, an issue that Ms. Warren had made a central part of her 2020 presidential campaign. They argued together that families desperately need relief from the high costs of child care.

Ms. Warren said that Mr. Mamdani had the right affordability message for Democrats nationally.

“New York City is the place to start the conversation for Democrats on how affordability is the central issue — the central reason to be a Democrat — and delivering on it in meaningful, tangible ways that will touch working families,” she said.

Then she laced into Mr. Cuomo, accusing him of being beholden to wealthy donors and slow to understand everyday New Yorkers’ worries about the cost of living.

“Andrew Cuomo is spending his campaign evidently out just grabbing up as much money as he can from millionaires and billionaires who are unhappy” about Mr. Mamdani’s candidacy, she said, adding that Mr. Mamdani “didn’t start by bending a knee to the billionaires to say, ‘What would make your life better?’”