Cynthia Nixon’s insurgent bid to unseat Gov. Andrew M. Cuomo got a lift on Monday: The national arm of Indivisible, the liberal grassroots network that sprang up following the election of President Trump, endorsed her.

The endorsement occurred a day after Ms. Nixon announced she was backing Jumaane Williams, a New York City councilman from Brooklyn, for lieutenant governor, and he endorsed her, making the allies an official ticket.

Ms. Nixon and Mr. Williams are the underdogs in the contest and the backing of Indivisible, with 400 registered chapters in New York scattered all across the state, according to the group, provides them a potential network of supporters for her shoestring candidacy to tap into during the race’s final six-plus weeks.

The endorsement comes after Mr. Cuomo personally wooed at least some Indivisible leaders in recent weeks, including a lengthy meeting at Docks, the governor’s favored seafood haunt beneath his office in Midtown Manhattan.