Three days after endorsing Andrew M. Cuomo for mayor of New York City, Michael R. Bloomberg put his money where his mouth is.

On Friday, Mr. Bloomberg, the billionaire former mayor, gave $5 million to Fix the City, a super PAC that is supporting Mr. Cuomo’s bid for mayor. That brings the PAC’s haul to roughly $20 million. Even before this donation, it was the biggest super PAC in New York City history.

The new infusion of resources will amplify the PAC’s ability to target Zohran Mamdani, the democratic socialist candidate now perceived as Mr. Cuomo’s main target in the June 24 Democratic primary.

Before Mr. Bloomberg’s donation, the PAC had raised roughly $15 million, according to a spokeswoman, and spent about $9 million, much of it on advertising seeking to draw a contrast between a purportedly can-do Mr. Cuomo, 67, and Mr. Mamdani, a 33-year-old state assemblyman from Queens who is described in the ads as a “risk.”

The donations came days after an apparently draft piece of PAC campaign literature was leaked to a reporter showing an altered photo of Mr. Mamdani that made his beard appear thicker and darker, next to language suggesting he was anti-Jewish. The PAC said it rejected the proposed literature and that it was never sent out. Mr. Mamdani, who is Muslim, described the literature as Islamophobic.

Mr. Bloomberg’s donation will allow Mr. Cuomo’s allies to flood the airwaves just as early voting begins on Saturday. Some of his rivals have super PACs, though none have raised anywhere close to even Mr. Bloomberg’s individual contribution. New Yorkers for Lower Costs, a super PAC created to support Mr. Mamdani, has reported raising $400,000.