Via Angela in the comments, Dr. Robert Stackpole posted a series of essays...
...which contain his thoughts on how Catholics ought to vote, given Obama's unreserved support for abortion rights and McCain's problematic positions on other issues.
I come to a different conclusion than Dr.Stackpole, but I respect his decision and don't question the sincerity of his arguments.
As one of the commenters to part 3 said - there is plenty of reason to be skeptical of Republican's true commitment to opposition to abortion, leaving aside the notion that opposing abortion does not, in and of itself, make one "pro-life." They've had 8 years: remember the big push to pass a constitutional amendment protecting all life from conception until natural death? Neither do I.
For that matter, after Roberts and Alito took their seats on the Supreme Court, why didn't the Republican-controlled Congress itself pass a law outlawing abortion, so that the "Pro-Life" Bush administration could then send the Solicitor General to argue for it in front of the Supreme Court? Republican Congress, Republican Administration: why no real action?
I'm convinced it's because they don't want to actually settle the abortion question - firstly because there is (unfortunately) a clear majority of public opinion that abortion should be legal in at least some circumstances, and thus actually outlawing it would present huge problems for them in terms of a backlash ( I have read some of the contingency planning the pro-choice movement has done for the event that Roe is overturned; the backlash would be absolutely massive, trust me); and secondly because that would deprive them of an issue they can use to get elected. "Just give us the chance to nominate one more justice, and then - then! - we'll finally be able to outlaw abortion. I know you Pro-Life liberals hate our other policies, but you're morally obligated to vote for us because of this one over-riding issue."
Disingenuous in the extreme, in my view.
Ponder this: It occurs to me that real and substantial progress might be made on the abortion issue if pro-lifers did more outreach to Liberals (beyond just telling them to vote for Republicans - an obvious non-starter.)
Pro-lifers don't have a lot of growth potential with Republicans and conservatives: the ones who can be convinced, have been. So, why doesn't the Pro-Life movement reach out to liberals? You know, have a fellow liberal explain opposition to abortion in a way that makes sense to the world-view of your typical Democrat, using progressive world-views and framing. I do this myself: there is lots of growth potential here, believe me. (Note the link to the right to Democrats For Life.) Support for abortion is pretty soft among lots of the rank and file Democrats I'm acquainted with.