Democratic former Boston mayor endorses Scott Brown in new television ad

"I'm a Democrat, but I'm tired of all the polarization, the pettiness, the bickering," Flynn said. "Scott Brown is a person you can work with. I've been in politics for 50 years, and that's really the name of the game, electing someone you can trust."

Flynn served as mayor of Boston from 1984 to 1993, and was appointed by former President Clinton as ambassador to the Vatican. He lost a congressional race against Joseph Kennedy (D) in 1998.

At the same time, Flynn has a record of flirtation with Massachusetts Republicans that could undercut the impact of his endorsement. Flynn endorsed former President George W. Bush over Al Gore in 2000, and said earlier this year that he would endorse Mitt Romney. He also backed Brown in the 2000 special election against state Attorney General Martha Coakley.

On Thursday, Brown was endorsed by New York City Mayor Michael Bloomberg, who heralded his record on gun rights.

“The biggest reason the mayor is supporting Sen. 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,” Bloomberg spokesman Stu Loeser told The New York Times. "If you take a tough stand and buck party orthodoxy [in a way] that helps the city of New York, the mayor would like to support you.'’