In his column at WaPo‘s ‘The Fix,’ Chris Cillizza discusses Senator McCain’s latest veepstake trial ballon. Cillizza quotes McCain:

I think that the pro-life position is one of the important aspects or fundamentals of the Republican Party…And I also feel that — and I’m not trying to equivocate here — that Americans want us to work together. You know, [former Pennsylvania Governor] Tom Ridge is one of the great leaders and he happens to be pro-choice. And I don’t think that that would necessarily rule Tom Ridge out…I think it’s a fundamental tenet of our party to be pro-life but that does not mean we exclude people from our party that are pro-choice. We just have a — albeit strong — but just it’s a disagreement. And I think Ridge is a great example of that.

Cillizza sees it as a genuine veepstakers trial baloon. My guess is that what McCain is really up to here is trying to cool out pro-choice GOP women and PA Republicans, since he still harbors hope that he can be competitive in that state. Ridge will almost certainly get a big cabinet job if McCain wins. But putting a pro-choice veep on the ticket could open up a big can of ugly at the GOP convention, with angry ‘pro-lifers’ giving bitter interviews to anyone who will listen.
Also, Seema Mehta and Maeve Reston caught a revealing comment by McCain in their L.A. Times post, “Sounds like Tom Ridge is out of the VP picture” :

…Conservatives scoffed at the notion that McCain, already viewed with suspicion in some conservative circles, would choose a running mate who supports abortion rights. And McCain hinted Monday, perhaps unintentionally, that Ridge might not be on his short list.
During a visit to a General Electric plant in Erie, Pa., with Ridge in earshot, McCain was asked what he would do in his first 90 days in office. He replied, “Call Tom Ridge to Washington from whatever vacation he’s taking and get him to work.”

McCain is clearly worried about losing the votes of pro-choice women in the general election, even though he has cast his lot with a strong anti-choice position. He doesn’t want to discuss his position regarding criminalizing abortion in any detail because it could be a huge loser, with the rapidly-growing single women demographic and what polls show about their views on abortion-related issues.
Dems have much to gain by making sure the public, especially women swing voters, understand that he favors a constitutional amendment banning abortion, overturning Roe v. Wade and more Supreme Court justices who support criminalizing abortion. At some point during the campaign, he should be asked straight out “Do you favor criminal penalties for women who have abortions?” Letting him get to November 4 without answering this question on camera would be the real crime.