Of all the polarizing things Mayor Michael R. Bloomberg has said and done over the years, from banning large sugary drinks to supporting congestion pricing, few have generated the sort of viral backlash that has unexpectedly mounted after his weekly radio show on Friday.
The mayor had embarked on a lengthy stream of consciousness on the need to negotiate a new teacher evaluation plan with the United Federation of Teachers. Toward the end, Mr. Bloomberg, almost as an aside, likened the teachers' union to groups like the National Rifle Association and others in which he said a few leaders were out of sync with large numbers of rank-and-file members.
"It's typical of Congress, it's typical of unions, it's typical of companies, I guess, where a small group is really carrying the ball and the others aren't necessarily in agreement," Mr. Bloomberg said to the program host, John Gambling. "The N.R.A. is another place where the membership, if you do the polling, doesn't agree with the leadership."
The comparison drew somewhat of a muted response. Michael Mulgrew, the president of the union, issued a one-sentence statement later on Friday saying the remark was "completely inappropriate" and indicative of the continuing difficulty in the union's relationship with the mayor.
But by Sunday, 70 people had signed a letter by the union to the mayor, which said the union was among the first to give counseling and other help to those in Newtown, Conn., after the school shootings there on Dec. 14.
Among those who signed the letter were most of the presumptive Democratic mayoral candidates. Christine C. Quinn, the City Council speaker, had not signed it as of Sunday evening, but her aides said she intended to.
And on Monday, the union was scheduled to hold a news conference to underscore its outrage and to demand that the mayor apologize.
This was not the first time Mr. Bloomberg had invoked the National Rifle Association when registering his frustration with the city teachers' union and others. He voiced similar opinions at a news conference in 2007, in the face of resistance to part of his educational agenda at the time.
"As the mayor has said before, the union is a special-interest group focused on advancing its agenda, whether it's in the public interest or not," Deputy Mayor Howard Wolfson said in a statement on Sunday. "Their refusal to agree to a fair evaluation deal is just the latest example of this."
Randi Weingarten, the president of the American Federation of Teachers and the former president of its New York City affiliate, wrote a separate letter to the mayor, noting that two of the adults killed in Connecticut were members of her union. She said that despite Mr. Bloomberg's "great work" on gun safety, he owed the city teachers and its union leaders an apology "for making such incendiary and insensitive remarks -- especially coming on the heels of the tragedy in Newtown."
This article originally appeared in The New York Times.