New York Mayor Michael Bloomberg is throwing his considerable political — and financial — heft behind Massachusetts Republican Scott Brown's re-election bid against liberal consumer advocate Elizabeth Warren, upping the ante in what is already the most high-profile and expensive Senate race of the 2012 election.

Politico reports that the billionaire media mogul/mayor will host a fundraiser for Brown at his Upper East Side townhouse next month, one of the first such events he has hosted this year. Bloomberg typically backs several candidates each cycle from across the political spectrum.

Bloomberg's endorsement of Brown is perhaps unsurprising, given Warren's aggressive stance toward Wall Street reform, a position that helped launch her political star. Bloomberg, on the other hand, has emerged as one of the staunchest defenders of the financial services industry, and tends to dismiss bank criticism as naive.

But the Mayor's office insists that his Massachusetts Senate endorsement is not about Warren or Wall Street, but about Brown's willingness to buck his party on gun control. Bloomberg has been a longtime advocate for more restrictive gun laws, and is one one of the loudest voices on the issue in the wake of last week's mass shooting in Aurora, Colo.

“If you take a tough stand and buck party orthodoxy that helps the City of New York, the mayor would like to support you,” Bloomberg spokesman Stu Loeser told The New York Times, dismissing criticism over Brown's high NRA ratings and lack of support for a new federal assault weapons ban.

“The biggest reason the mayor is supporting Senator Brown is the senator’s help on one of our biggest gun issues: Opposing concealed-carry reciprocity that would let people with gun permits from rural states like Arkansas and Kentucky carry hidden handguns in New York City," Loeser said.

When asked whether Warren's Wall Street positions played any role, however, Loeser danced around the question:

"If people say the challenger is better on X, Y or Z, it’s not a question of whether they are or aren’t. We tend not to entertain the hypothetical over the real.”