Michael R. Bloomberg, the billionaire mayor of New York, waded into one of the nation’s fiercest Senate races and threw his financial and rhetorical support behind Senator Scott Brown on Thursday, praising Brown’s defiance of the National Rifle Association on its top legislative priority.
The “biggest reason” the mayor decided to support Brown was the senator’s opposition to the National Right-To-Carry Reciprocity Act, which would allow gun owners to carry concealed weapons across state lines, said Stu Loeser, a Bloomberg spokesman.
Bloomberg, who cofounded a group called Mayors Against Illegal Guns with Mayor Thomas M. Menino of Boston, has made gun control one of his top issues.
The New York mayor, who will hold an August fund-raiser for Brown at his Upper East Side townhouse, could help the senator tap wealthy New York donors and build his case that he is an independent voice willing to buck party orthodoxy.
Bloomberg is an independent who was previously a member of both major parties.
He is perhaps the nation’s most prominent advocate for a brand of centrist politics that mixes support for business with liberal social views and tough regulation of some industries.
At the same time, the endorsement could make it more difficult for Brown, a Massachusetts Republican who prides himself on his humble, homegrown image, to criticize his Democratic challenger, Elizabeth Warren, for raising money out of state.
The New York mayor could help the senator tap wealthy New York donors and build his case that he is an independent voice.
Whether voters will care is another question; political scientists often dismiss the importance of endorsements, saying they can burnish a candidate’s imagine, but do little to sway votes.
Nevertheless, Warren, who accepted the endorsement of the Massachusetts Credit Union League on Thursday, pounced on Bloomberg’s backing as evidence that Brown champions the interests of major financial institutions.
“Today, Scott Brown stands with Wall Street, and I stand with every credit union in the Commonwealth,” she said at her campaign headquarters in Somerville.
A vocal defender of Wall Street, Bloomberg may chafe at Warren’s persistent criticism of the financial sector and big banks.
But Loeser, the Bloomberg spokesman, said that was not the basis for the endorsement and declined to comment on Warren’s candidacy.
Brown drew financial support from the National Rifle Association in his 2010 run for US Senate and received an A+ grade from the Gun Owners Action League in 2008, when he was a state senator.
But under pressure from Menino and Bloomberg, he announced last November that he would oppose the Reciprocity Act, saying states should be allowed to set their own gun laws.
“In Albany and Washington, when legislators take a tough vote for the people of New York, against their party’s or special interests’ orthodoxies, the mayor appreciates it and is inclined to show his support,” Loeser said.
Brown’s campaign welcomed the mayor’s endorsement.
“Mayor Bloomberg is a true independent who always puts progress over politics,” spokesman Colin Reed said in a statement Thursday. “That’s Scott Brown’s philosophy as well, and he is honored to have the mayor’s support.”
Bloomberg first met Brown two years ago when he lobbied Brown and other senators to support a bill that provided compensation for workers sickened at the site of the World Trade Center attacks.
Though the two have not built much of a personal relationship, Loeser said the mayor respects Brown’s vote to repeal the federal ban on openly gay members of the military and the senator’s support for the Violence Against Women Act, which toughens domestic violence laws and funds training for police.
On gun issues, Bloomberg may be more closely aligned with Warren.
The Harvard Law School professor favors more restrictions than Brown, including an extension of the federal assault weapons ban, which expired in 2004 and would have prohibited the kind of assault rifle used in the Colorado theater shooting last week.
But Loeser said that, in general, the mayor likes to support legislators who take tough votes, rather than challengers who promise more.
“We don’t follow the argument that, hypothetically, someone else would have done better,” he said.
Bloomberg has an eclectic history of endorsements. The mayor, who grew up in Medford, backed Alan Khazei in the 2009 Democratic primary for the US Senate seat in Massachusetts that was left vacant by Edward M. Kennedy’s death and that Brown later filled.
In 2010, he supported Meg Whitman, the eBay billionaire and Republican who ran unsuccessfully for governor of California, and Lincoln D. Chafee, the Republican turned independent who was elected governor of Rhode Island.
That year, he also rallied to the defense of Harry Reid, the Senate majority leader and Nevada Democrat who was fending off a strong challenge from Sharron Angle, a Tea Party-backed Republican.
This year, President Obama and Mitt Romney have each courted Bloomberg’s endorsement.
But the mayor, who is often said to harbor his own presidential ambitions, has declined to support either one.
While Brown basked in Bloomberg’s support on Thursday, Warren welcomed Debbie Wasserman Schultz, the chair of the Democratic National Committee, to her Somerville headquarters.
Schultz said Bloomberg’s endorsement does not mean Brown is a moderate. Massachusetts voters need to know “their senator pretends to be one person when he’s in the state, and he’s an extremist embracing the right wing when he’s in the nation’s capital,” she said.
On Friday, Brown plans to unveil a new television ad that seeks to fight such attacks. The ad, which will run during the Olympics, features Raymond L. Flynn, the former Democratic mayor of Boston, who has also endorsed Romney for president, praising Brown as honest and hardworking.
“I’m a Democrat but I’m tired of all the polarization, the pettiness, the bickering,” Flynn, seated in his South Boston living room, says in the ad. “Scott Brown is a person that you can work with.”Levenson can be reached at firstname.lastname@example.org. Follow him on Twitter @mlevenson. Noah Bierman of the Globe Staff contributed to this report.