Thanks to the current Lieberman-for-veep boomlet (and the very interesting news that Karl Rove has been trying to smother it in its crib), there's been a fair amount of discussion about exactly how much John McCain would hurt his standing among pro-lifers if he picked a pro-choice running mate such as Lieberman or Tom Ridge. The more interesting question to me, though, is whether such a pick might actually hurt McCain among pro-choicers as well.
For the moment, McCain has to some degree the best of both worlds: a strong pro-life record to peddle to conservatives, and the widespread sense among liberals and moderates--vividly demonstrated by clueless Clintonite-turned- McCainiac Debra Bartoshevich--that he's kinda sorta pro-choice, or at least ambivalent on the subject. (George W. Bush rode a similar strategy of courting conservatives through targeted appeals and surrogates, while maintaining a broader image of moderation, straight to the White House in 2000). What McCain presumably wants, then, is for the subject of abortion to come up as little as possible: The more he's forced to talk about it, the more he'll inevitably upset this delicate balance.
If he picks a pro-choice running mate, however, he'll have to talk about abortion a lot. There will be protests by pro-lifers and demands that he declare, loudly and repeatedly, his commitment to the pro-life cause in general and to placing anti-Roe justices on the Supreme Court in particular. He will have to make abundantly, emphatically clear that he, and not his pro-choice veep, will be making any and all executive decisions regarding abortion. He will be forced, in short, to dynamite his calculated ambivalence. We won't know how this might play out unless and until McCain actually picks a pro-choicer-- and the smart money still says he won't--but my guess is that it could cost him at both ends of the spectrum.