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Two leading candidates in the 2021 race for mayor of New York City are already beginning their ground game.

Corey Johnson, the speaker of the New York City Council, and Scott M. Stringer, the city’s comptroller, will soon begin hosting fund-raising house parties, seeking to plant a stake in what is certain to be a prolonged race for City Hall.

Neither Mr. Johnson nor Mr. Stringer has formally declared his candidacy, but the small, intimate gatherings were designed with the city’s new campaign finance rules — which increase the power of small donations — in mind.

Mr. Johnson said that a year ago he had no plans to undertake an underdog run for mayor in 2021.

“I love this city. I love New York,” Mr. Johnson said in an interview with The New York Times.