Ms. Gillibrand has been alone among Senate Democrats in voting against all but one of the president’s cabinet picks and posts requiring Senate confirmation (she voted in favor of Nikki R. Haley’s appointment as United States ambassador to the United Nations). Senator Elizabeth Warren of Massachusetts, to the disappointment of many of her admirers, advanced Ben Carson as housing secretary. Young progressive women in New York who have suddenly been making activism as much a part of their daily habits as chia seed pudding have become infatuated with Ms. Gillibrand, who has long drawn the adoration of the Sheryl Sandbergs of the world — members of the celebrity-feminist corporate class that we now know women seeking the presidency cannot rely on solely.

A few weeks ago, at a convening of hundreds of Brooklynites organizing to counter the Trump administration, I met a 30-year-old public school science teacher named Liat Olenick who lived in Bedford-Stuyvesant and, though never having been particularly political, was now going to three or four protests a week, and spending what time remained calling elected officials every day. Early on she cold-called Ms. Gillibrand’s Manhattan office to set up a meeting with staff members to talk about the fight against the cabinet choices. She was shocked to have her phone call returned the next day, and to get a meeting shortly thereafter. She said it took a month and a half of back and forth to elicit the same response from Senator Chuck Schumer’s office.

“My initial impression of Gillibrand was that she was a moderate voice in the Senate,” Ms. Olenick told me. “When I thought about the firebrands who could lead the opposition, she wasn’t one of them. But I think people are feeling really surprised and hopeful about her. She has a remarkable composure and thoughtfulness and she definitely has the appearance of sincerity.”

It is precisely that impression — that she isn’t scheming, that she isn’t tacking left simply for political utility, even if, in fact, she might be — that could put her ahead of Gov. Andrew M. Cuomo, a mentor, in a race for the Democratic presidential nomination in 2020. Both have been talked about as prospective candidates. Liberals like the governor more than they used to, and he has seemingly become more progressive, standing up for a $15-an-hour minimum wage and behaving as a champion of the marginalized, a warrior against future and present Trump-based indignities. On Saturday night, when people flocked to Kennedy International Airport to protest the executive order on immigration, the governor ordered the Port Authority to reverse its decision to allow only those holding plane tickets to board the AirTrain, proclaiming that the voices of New Yorkers would be heard. Still, he has never shaken the impression that expedience is his first calling.

Ms. Gillibrand is honest about what she did and didn’t know at the beginning of her tenure in the Senate. She is willing to admit her mistakes and the gaps in her knowledge. Swelling support for her comes at a time when her closest colleague, Mr. Schumer, a fellow Democrat who is the Senate minority leader, has been the target of repeated protests from constituents who are either aggrieved over his approval of some of Mr. Trump’s cabinet nominees or who simply want to let him know they expect him to lead the resistance without wavering.