Ms. Miner plans to run under the banner of an upstart new group, the Serve America Movement, which calls itself SAM, formed by people disaffected by the existing party structure after the 2016 elections. She will be the group’s first candidate.

“Stephanie is, from our perspective, the vanguard, the pioneer, the first one to go,” said Scott Muller, a leader of SAM and a former general counsel of the C.I.A. He said the plan is for Ms. Miner to petition to create a SAM Party in New York, with the eventual goal of a national party presence.

“We are going to do everything we can do under the law to help,” Mr. Muller said. “She will be our first priority.”

Mr. Molinaro issued a statement welcoming Ms. Miner into the race. “This is now a four-way contest,” he declared.

Ms. Nixon said that she and Ms. Miner were running different races. “She’s more of a moderate and I’m definitely a part of the progressive wing of the Democratic Party,” Ms. Nixon said. Mr. Cuomo’s campaign declined to comment.

For months, Ms. Miner has publicly flirted with running for governor, though few political insiders seemed to take her particularly seriously, given Mr. Cuomo’s strength as a two-term incumbent and Ms. Miner’s previous dalliance with a potential run for Congress.

Throughout, Ms. Miner has said that her decision would not be affected by Ms. Nixon’s run. But Ms. Nixon and Ms. Miner met privately this year, before Ms. Nixon announced her bid, when they and their spouses shared a meal in Syracuse, according to two people familiar with the meeting.