This is despite the fact that his answer to "Are you pro-choice" is arguably just about the most liberal version of the staunchly pro-choice answer.
Asked if he is pro-choice, Brown said: "Yes, because I feel this issue is best handled between a woman and her doctor and her family."
Notice the absence of any religious figure or institution (or God), which is what NARAL Pro-Choice America says: "This decision is a personal one between a woman, her family, her doctor, and her God"
Still, though, I suppose they felt that, for one, any Republican holding a seat is better (this, along with Casey's support for birth control, led Planned Parenthood to urge votes "against Rick Santorum" in 2006; he did vote to confirm Sotomayor as well last year) than a Democrat, and that maybe Brown would vote for (or oppose overturning) the various restrictions in place, while Coakley would not. Additionally, I suppose Brown may be likely to oppose taxpayer funding of abortions anyway. But it is still a big compromise on what "Right to Life" groups consistently (and very offensively) refer to as a "Holocaust of the unborn"
However, the most interesting thing is how much Massachusetts has changed on the abortion issue in the last 35 years.
Back in 1976, Henry Hyde introduced his amendment banning taxpayer funding of abortions for the first time. It passed narrowly, 207-167.
Of course, the House had 291 Democrats and 144 Republicans at the time, but on the other hand, the proportion of Democrats outside the greater South (former Confederacy, Kentucky and Oklahoma) is higher today (68.2% vs 64.7%)
In Massachusetts, though, 2 of the 3 Democrats opposing it didn't even have to worry about an abortion affecting them because they didn't have sex with women; priest Bob Drinan and then of course Gerry Studds.
Of course, 113 Democrats voted against it and 33 Republicans voted for it, so things weren't nearly as party line (even more so on the Democratic side, where if I counted properly, the majority against it was rather narrow, 134 to 113).
Still, things have changed if the Right to Lifers will endorse someone with as blanket a pro-choice statement as Brown under any condition.