Ms. Marinoni has proved as wary of the political press as of the celebrity, and in a rare interview, she downplayed her involvement in the campaign. As first lady, she said, she would remain in the background too, anchoring the family while Ms. Nixon governed.

“She’s been approached by folks repeatedly — not me, it was never my idea — who were excited about the prospects,” Ms. Marinoni, who has no official role in the campaign, said of Ms. Nixon’s decision to enter the race.

As the wife of a celebrity, she said she had perfected the art of dodging attention. “I now know how to step in or out of the spotlight, and I choose generally to stay out.”

But spotlight or not, people who have been involved with the campaign described Ms. Marinoni as an essential force in it: corralling support, shaping strategy, even serving as a proxy of sorts for her wife during policy or tactical discussions when Ms. Nixon is on the trail.

“Cynthia’s the candidate, and she’s kind of the embodiment of what we’re all trying to move on a state level,” said Jonathan Westin, the executive director of the nonprofit New York Communities for Change, which asked Ms. Nixon to run. “But the actual putting it together — I mean, Christine has been irreplaceable.”

Ms. Marinoni is a frequent presence at campaign events, marching alongside Ms. Nixon in the New York City Pride parade or collecting signatures to secure her spot on the ballot. After Ms. Nixon’s debate with Mr. Cuomo, Ms. Marinoni strode through the press room to make sure reporters got the story right: “She kicked ass! Clearly won. Clearly won.”