Mayor Michael R. Bloomberg, in a sharp escalation in the battle over gun control, is seeking to punish Democratic senators by taking away the one thing they most need from New Yorkers: money.
On Wednesday, Mr. Bloomberg will send a personal letter to hundreds of the biggest Democratic donors in New York urging them to cut off contributions to the four Democratic senators who helped block a bill in April that would have strengthened background checks on gun purchasers.
The move could inflame tensions that have simmered for weeks between Mr. Bloomberg, who blames the four Democrats for the defeat of the bill, and Democratic Senate leaders, who have privately told City Hall that the attacks can serve only to empower a Republican majority openly hostile to Mr. Bloomberg’s priorities.
By appealing to the Democrats’ financial base, Mr. Bloomberg is exploiting his relationships and prestige among wealthy New Yorkers to disrupt the flow of campaign money to key Democrats whose re-election next year will help determine whether the party retains control of the Senate. No state is more essential to the party’s fund-raising: Sitting Democratic senators and the Democratic Senatorial Campaign Committee raised $30.4 million from New York donors in 2012, according to the Center for Responsive Politics, more than in any other state.
And the four Democratic senators who sided with Republicans filibustering the background check bill — Mark Pryor of Arkansas, Max Baucus of Montana, Mark Begichof Alaska and Heidi Heitkamp of North Dakota — have raised more than $2.2 million from New York.
In an interview, Mr. Bloomberg said he believed gun deaths had reached such a state of crisis that he needed to force the issue.
“If they come and ask for the money, you say to them, ‘What do you stand for?’ ” he said. “I want to tell people what these four stand for. And then people can make up their own minds.”
Mr. Bloomberg’s strategy creates a tricky situation for Senate Democrats. They do not wish to alienate the billionaire mayor, who has become increasingly aggressive and outspoken on the issue. But they say he should be more sympathetic, given that their party, with its fragile majority, has tried to take on the difficult subject of increasing restrictions on guns in the face of hostility from Republicans.
“What they are doing,” said one senior Democratic aide who, like many people interviewed for this article, declined to go on the record criticizing the mayor, “is increasing the likelihood of a 100 percent A-rated N.R.A. Republican being elected.”
For Senator Charles E. Schumer, the New York Democrat who was an architect of his party’s takeover of the Senate in 2006 and has aggressively built up the Democratic donor base in New York, and who is a close ally of the mayor, the notion that Mr. Bloomberg’s efforts could cost Democrats their majority is not an abstraction.
He praised the mayor’s effort, but added, “We should be mindful that pro-gun safety laws have a much better chance of passing under a Democratic Senate majority than a Republican one.”
Mr. Bloomberg’s idea to use his formidable resources against Senate Democrats was enough of a threat to the majority leader, Harry Reid, that he raised the issue with the mayor in Washington in February.
According to a person with direct knowledge of their conversation, Mr. Reid told the mayor that he thought any efforts to attack Democrats would be shortsighted, and could ultimately result in a Republican Senate majority.
“Do you think you’d be better served by Majority Leader McConnell?” Mr. Reid said, according to this person, referring to Mitch McConnell of Kentucky.
Mr. Bloomberg appears undeterred. In May, he financed a $350,000 blitz of tough television ads against Mr. Pryor, featuring the fatal shooting of a friend of the senator. Mr. Bloomberg said voters would reward the senators if they heeded the public — which broadly favors background checks — instead of the National Rifle Association and other groups opposed to the legislation.
“If Democrats want to keep control of the Senate, what I would suggest is that they have all of their members vote for things that the public wants,” he said in the interview. “And if they don’t do that, the voters should elect different senators who will listen to them. That’s what democracy is all about.”
If a background-check bill passes the Senate, Mr. Bloomberg said, he will seek to put similar pressure on the Republican-led House, including appeals to Republican donors in New York who favor more gun regulation. Some Senate Republicans have already found themselves targets of the mayor.
But his latest move focuses more on Senate Democrats because, aides said, none of the Republican senators who voted to block the gun bill appear to be facing competitive re-election races in 2014.
That has put Mr. Bloomberg in the unusual position of applying his pressure mainly on the party that most broadly shares his views on gun control and other policy issues.
Privately, Democratic senators and aides complain that the shaming, scolding tone of some of Mr. Bloomberg’s ads is counterproductive. The mayor’s group has accused senators who voted against background checks of betraying their constituents and distorting the facts on gun control, an antagonism that many Democrats fear will only make the “no” votes firmer.
Democrats say they would prefer an approach from Mr. Bloomberg that is more positive, and aimed not at the four Democrats, but at gun owners who support tighter restrictions and can be moved to pressure their senators.
“Why wouldn’t you try to educate them?” said Senator Joe Manchin III, the West Virginia Democrat who helped write the background check bill. He added that he could empathize with the mayor’s frustration. “I understand the process is tough,” Mr. Manchin said. “He’s probably very upset like a lot of us.”
In response to Mr. Bloomberg’s ad, Mr. Pryor filmed his own, in which he adopts a defiant tone. “The mayor of New York City is running ads against me because I opposed President Obama’s gun control legislation,” Mr. Pryor says in the ad.
In an interview, Mr. Begich, who, like Mr. Pryor, faces re-election next year, said he was unbowed by the threat of a Bloomberg-led attack. Indeed, he seemed to almost relish the thought of one.
“In Alaska, having a New York mayor tell us what to do? The guy who wants to ban Big Gulps?” Mr. Begich asked incredulously. “If anything, it might help me,” he added.
Still, the threat of a campaign bankrolled by a man with virtually endless financial resources — and, come 2014, a lot more time on his hands — is palpable in the cloakrooms of the Senate.
Senator Mary L. Landrieu of Louisiana, a Democrat who is also up for re-election in 2014 but voted for the background check bill, declined to second-guess the mayor. In March she was one of the targets of a $12 million advertising campaign by Mayors Against Illegal Guns, urging lawmakers to vote for the measure.
“I’m not going to comment on that because I have a lot of respect for Mayor Bloomberg,” she said. “And he most certainly is a wise-enough politician that he doesn’t need counsel from me. Period.”