"We consider him a senator who votes prolife," said Anne Fox, president of the Massachusetts Citizens for Life. "We have to take his word for it when he says he is prochoice. But what we're looking for is someone who votes prolife, and he does."

Brown said he would not reject the backing of Massachusetts Citizens for Life but reiterated that he considers himself a supporter of abortion rights.

"There are people of good will on both sides of the abortion issue, and Senator Brown welcomes their support, but he is prochoice and his record going back many years reflects that," said Colin Reed, a Brown spokesman.

Still, Brown received the backing of Massachusetts Citizens for Life in his 2010 run for Senate and has sided with the National Right to Life Committee on four of its five key votes since he arrived in Washington, earning him an 80 percent approval rating.

Meanwhile, Brown rebuked this year's GOP "no exceptions" national platform plank on abortion. "I believe this is a mistake because it fails to recognize the views of pro-choice Republicans like myself," he wrote to party chairman Reince Priebus. "Even while I am pro-choice, I respect those who have a different opinion on this very difficult and sensitive issue. Our party platform should make the same concession to those of us who believe in a woman's right to choose."

But here is Flynn explaining his endorsement to the Globe:

Although Brown has repeatedly said he favors abortion rights, Flynn cited Brown's endorsement by Massachusetts Citizens for Life, the state's leading anti-abortion group. In the US Senate, he cosponsored the Blunt Amendment, which would allow health plans and employers to refuse to provide or pay for services such as abortion that they oppose on religious or moral grounds.

"He was endorsed by Massachusetts Citizens for Life," Flynn said. "That's an organization I believe in. That means there must be something there, that he has a lot of respect for life and the needy."

Neil Sullivan, who was chief policy adviser to the former mayor, described Flynn's backing of the Republican as "a single-issue endorsement."

"I view his endorsement of Scott Brown as Ray Flynn, citizen for life," Sullivan said.

And yet, Scott Brown is running an ad in which he stresses that he is prochoice.  (I live in MA and I have seen it a lot lately.)

That Flynn has never had a nuanced view on the matter is underscored by his long term relationship with Fr. Frank Pavone, with whom Flynn dined when  Pavone visited Boston last year.  A few months later Pavone wrote:

"When I preach - and help other priests to preach - the clear message that candidates and parties must defend life, some - including clergy - complain to me that my message hurts their favorite candidate or party. My response? `Go tell your favorite candidate or party to get the babies' blood off their hands and clean up their act regarding defending life. Then my words won't hurt them anymore.' "

And this year, Flynn endorsed a prochoice politician.