In the days leading up to the release of ratings for thousands of New York City public-school teachers on Friday, hundreds of e-mails poured into the in-box of Michael Mulgrew, president of the United Federation of Teachers.

“Enough of cooperation,” one member of the union wrote to his leader. Others prodded Mr. Mulgrew to stand up against Mayor Michael R. Bloomberg, describing him as “untrustworthy,” in what he said was a call to arms of unparalleled intensity.

“How many times do we have to get kicked in the teeth before we realize we can’t work with these people?” John Elfrank-Dana, a union chapter leader at Murry Bergtraum High School in Lower Manhattan, asked during an interview, echoing what many of his fellow teachers have said in recent days on Twitter and on various blogs.

Mr. Mulgrew and his comrades had fought for more than a year to block release of the ratings, known as teacher data reports, which try to calculate how much value individual teachers add by predicting their students’ test scores and then measuring how much they exceed or fall short of those expectations. But the legal defeat a court dealt the union, by green-lighting the release, may yet be a political victory for the union — by galvanizing members and mobilizing allies on the left, including the Occupy movement andChange.org, through which scores of people signed petitions and sent letters to news organizations last week protesting the publication of the ratings.

“There’s brinkmanship from all sides, but from a political standpoint, Mulgrew is certainly the strongman, even if, from a legal standpoint, it’s City Hall that has the upper hand,” said David C. Bloomfield, a professor of education at Brooklyn College.

Mr. Mulgrew, who has spent the past three years building a case against Mr. Bloomberg’s education agenda of closing failing schools and promoting charters, sensed the opportunity.

“What I’m going to do now,” he said in an interview over the weekend, “is to stop the mayor from doing any further damage to the children of New York City.”

The posture has its risks: there is a lot of money at stake, for the city and for teachers. The Bloomberg administration and Mr. Mulgrew’s union are in the midst of negotiating the details of a state-mandated overhaul of teacher evaluations that would use the state test scores on which the controversial rankings are based, as well as subjective measures and possibly other exams. If they fail to come to an agreement by January, the city stands to lose some $200 million in state education aid, under a plan concocted by Gov. Andrew M. Cuomo to compel a compromise.

Already, whatever last bit of hope City Hall had of striking a deal on a new teachers’ contract before the end of Mr. Bloomberg’s term has all but evaporated, with Mr. Mulgrew focused instead on who might replace the mayor after the 2013 election.

The data reports, and the larger issue of teacher evaluations, could well become a litmus test for the Democrats already fighting for the union’s crucial endorsement.

Some of them rushed to condemn the rankings’ release on Friday, though in different ways.

Bill de Blasio, the public advocate, mimicked the union’s line of criticism, saying, “The mayor’s persistence in denigrating teachers is completely at odds with our need to move New York City forward by attracting the best and brightest to the profession.” Christine C. Quinn, the City Council speaker, on the other hand, focused her fire on the courts, for siding against the union in its suit to block the release.

By positioning himself as the unbendable leader of the aggrieved, Mr. Mulgrew has at once bolstered his profile among the union’s rank and file and backed himself into a corner. One way or another, the state must put in place a system to judge the quality of its teachers and principals, as required by legislation passed in 2010 as part of its successful application for $700 million in federal education aid through the Race to the Top program. The alternative would be to forfeit it, a path Mr. Cuomo has already said he will not accept.

Mr. Bloomberg has given no indication that he is willing to budge on his plan to close and reopen 33 struggling schools so the city can dismiss half of their teachers. The plan is an effort to restore federal improvement grants suspended because of the lack of an agreement on the evaluation system.

Trying to capitalize on the attention and momentum that built over the data reports last week, the union plans to rebroadcast a television advertisement starting on Monday that first was shown earlier this month. In it, Mr. Mulgrew looks into the camera and says, “Work with us for better schools and a brighter future for all our students.”

Though many critics last week assailed The New York Times and other news organizations for publishing the ratings, the union has made the mayor its primary target, mostly shrugging off the role of the news media. That is a contrast to the union’s counterpart in Los Angeles, which picketed outside the Los Angeles Times building after the newspaper published teachers’ names and ratings in 2010.

The circumstances were different in Los Angeles — the newspaper had hired its own statistician to devise the rankings, while in New York, the city itself had compiled them and used them in tenure and other decisions. But the political lessons may be transferable.

While the Los Angeles Unified School District now produces its own teacher rankings, it has declined to release them with teachers’ names attached, citing potential harm to school employees. And the union has steadfastly declined to agree to using so-called value-added scores as a factor in a new evaluation system.

In New York, the state’s new evaluation system would use similar measures to calculate at least 20 percent of a teacher’s score; it took more than a year of fighting in court and at the negotiating table for state officials and union leaders to agree on the value-added weight. Mr. Cuomo had to intervene, and he ended up drafting Mr. Mulgrew to help bridge the differences between both sides.

When the deal was announced in Albany on Feb. 16, praise for Mr. Cuomo came from all corners, including Mr. Mulgrew and some of the same mayoral hopefuls who are now criticizing the rankings’ release.

Now, amid the controversy of last week, more questions are stirring about the reliability of the new system, which was written into Mr. Cuomo’s budget but still has to be signed into law.

Merryl H. Tisch, chancellor of the Board of Regents, said publication made it “more complicated to go back and negotiate at the local level,” a requirement for the new system’s adoption statewide. Mr. Mulgrew’s predecessor, Randi Weingarten, who is now president of the American Federation of Teachers, said it “couldn’t have come at a worse time.”

The outcome largely depends on Mr. Mulgrew’s next move. He will have to either figure out a way to justify his support for the new system to the union’s angry membership, or withdraw it.