Mr. Bloomberg has agreed to host a fund-raiser at his Upper East Side town house for the re-election campaign of Senator Scott P. Brown, a Republican, who is being challenged by a fiery consumer advocate, Elizabeth Warren, a Democrat who is also a professor at Harvard Law School.

Mr. Bloomberg, through a spokesman, said his endorsement was not about Wall Street at all, but about his desire to reward Mr. Brown for voting against his party and the National Rifle Association on a gun control measure. …

Mr. Brown has generally received high ratings from gun rights organizations and has not pushed for renewing a federal assault-weapons ban or for tightening restrictions on gun shows. But Mr. Loeser said the mayor had been won over by the senator’s “tough stand” opposing a measure that would have required states to honor concealed-weapons permits issued by other states.