When New York Mayor Michael Bloomberg endorsed Massachusetts Sen. Scott Brown for re-election on Thursday, aides for "America's mayor" insisted the Republican senator's position on gun control -- not his professed support for financial and business interests -- is the primary reason Bloomberg is backing his campaign.

New York's billionaire mayor is a staunch gun control advocate (he co-founded Mayors Against Illegal Guns with Boston's Thomas Menino), an issue he essentially reintroduced to the national conversation following last week's move theater shootings in Aurora, Colo. So while it makes sense that he would lend his endorsement to someone who also champions that cause, critics are quick to point out that if gun control was truly the issue that swayed Bloomberg, he would have gone with the Democratic challenger, Elizabeth Warren, who favors tighter gun control laws than Brown.

For her part, Warren seems to believe Bloomberg's endorsement stemmed more from his interest in financial institutions rather than gun control. Bloomberg has been a vocal defender of Wall Street and the financial sector -- a stance that has earned the ire of Occupy Wall Street protesters -- while Warren, who organized the creation of the Consumer Protection Financial Bureau in the wake of the Great Recession, is critical of big banks and is a fierce supporter of financial regulatory reform.

"Today, Scott Brown stands with Wall Street, and I stand with every credit union in the Commonwealth," Warren said Thursday in reaction to news of Bloomberg's endorsement of Brown.

That same day Warren had accepted the endorsement of the Massachusetts Credit Union League.

Brown has been hailed as somewhat of a rogue Republican after he broke with most his party to oppose the National Right-to-Carry Reciprocity Act, which would allow gun owners to carry concealed weapons across state lines. On Thursday, Bloomberg spokesman Stu Loeser told the New York Times that vote is the "biggest reason" the mayor, a political independent, is backing Brown, "because if you take a tough stand and buck party orthodoxy that helps the city of New York, the mayor would like to support you."

And Bloomberg is definitely supporting Brown. Next month he's hosting a fundraiser for the Massachusetts Republican at his Upper East Side townhouse that could help Brown -- who has lagged behind Warren when it comes to fundraising -- tap into some wealthy New York donors.

But Bloomberg's endorsement is still questionable, considering it came less than a week after the mayor hammered President Barack Obama and his Republican challenger Mitt Romney over their decision to not immediately discuss gun laws in the aftermath of the Aurora shootings. Bloomberg, in an interview with CBS News, said prayers and sympathy over the tragedy were not enough and insisted that, "the candidates for the President of the United States [need] to stand up and say once and for all, yes, they feel terrible, yes it's a tragedy, yes we have great sympathy for the families, but it's time to stand up and do something."

How would Brown "stand up and do something"? Well, he probably wouldn't.

Yes, Brown opposed the National Right-to-Carry Reciprocity Act -- where he broke with the National Rifle Association, who contributed $59,000 to his 2010 special election, reportedly after heavy pressure from both Bloomberg and Menino -- and also supported Massachusetts' own assault-rifle ban while serving as a state representative in 2002 (it's worth noting he also voted against closing a "loophole" in the original law that still allowed for the sale and transfer of assault weapons owned prior to 1994). But he also believes gun control is an issue best handled individually by the states, a belief he cited when explaining why he opposed renewing the federal assault weapons ban, which expired in 2004.

Warren, on the other hand, favors an extension of the assault weapons ban, in addition to legislation requiring more rigorous background screenings -- including for people who purchase firearms at gun shows, who are typically exempted -- and opposes efforts to limit the sharing of firearms trace information with law enforcement agencies.

Meanwhile, Brown's campaign has been careful to avoid addressing the candidate's stance on gun control issues aside from the National Right-to-Carry Reciprocity Act. The Boston Globe reports that Brown spokeswoman Marcie Kinzel declined to answer several questions about the candidate's gun control policy in a recent interview, including his position on improved background screenings.

What we do know is Brown was a reliable vote for gun rights during his 12-year tenure in the Massachusetts House and Senate. The Globe reports he received an A-plus rating from the Gun Owners' Action League in 2008, although their support for Brown has cooled. A spokesperson for the national organization told the Globe they will not support Brown this election cycle, in part because of his "nuanced" position on gun rights.

But the gun lobby is doing quite well, even without Brown to lean on. The pro-gun lobby is one of the most influential on Capitol Hill. The National Rifle Association alone has spent nearly $25.6 million in campaign contributions and $24 million in lobbying expenditures since the late 1990s, the Sunlight Foundation reports, while the Gun Owners of America spent another $25 million lobbying Congress. Gun makers such as Smith and Wesson (NASDAQ: SWHC) and Remington Arms have also spent millions lobbying for gun rights legislation.