As a state lawmaker, Zohran Mamdani generally opposed plans to open full-scale casinos in New York City. In June, when he walked past a billboard showing betting markets for the city mayor’s race, he even quipped that “gambling is haram,” an Arabic word meaning forbidden.

But on Wednesday, Mr. Mamdani said that he did not intend to use his newfound political clout as the Democratic mayoral nominee and the race’s front-runner to try to stop the arrival of casinos that are widely considered a fait accompli.

Speaking alongside the leader of a union pushing the gambling expansion, Mr. Mamdani told reporters that while he still had doubts about the casinos backed by corporate giants, he respected the views of state voters who had approved the downstate gambling expansion more than a decade ago.

“I’ve been open about my personal skepticism, and yet I also know this is the law,” he said. “The siting and the choices of which casinos will open, that pertains to the state.”

Mr. Mamdani would be hard-pressed to stop the expansion even if he tried. The state process for awarding up to three casino licenses is scheduled to come to a head later this year, before the next mayor takes office on Jan. 1. The amount of money, momentum and political capital behind the various proposals renders the arrival of casinos all but unstoppable, even if opponents succeed in scuttling individual applications.

New York City is considered the last, great untapped gambling market in the United States, and there has been a fierce competition for licenses. Applicants have proposed eight multibillion-dollar casino developments in neighborhoods as diverse as Times Square, Coney Island and those near the United Nations in Manhattan and Citi Field in Queens.