Suppose we're convinced that a certain issue is more important than any other, and it's on the level of urgent moral necessity to do whatever we can to make progress on that issue and that issue, even if it sets us back quite a ways on other issues. I don't think that's true of the issue of abortion. Having pro-life leaders on the national level isn't better than having pro-choice leaders if the pro-life leaders are going to do things that are even worse than the status quo on abortion. I wouldn't vote for someone who thinks abortion is wrong if the person also thinks we ought to put the majority of the population in machines for eight hours a day that cause intense pain and shorten their lives conserably, merely to make the lives of a few elite people comfortable. While I think abortion is evil and unjust, I'd rather make little progress or even move backward on that score if it's a choice between that and moving into a society that's so bad that the abortion status quo pales in comparison. Those who tolerate grave evil are still better than those who would deliberately perpatuate a greater evil.
But even if we consider a certain issue to be so all-defining that we think we should care very little about anything else, I think we have a moral obligation to prefer someone who is closer to us on that issue than someone else who is further from us on the issue, even if we think both of them hold immoral views and are too tolerant of evil. This may well end up being the case with the 2008 presidential race for pro-lifers if it turns out that the two frontrunners get their respective party nominations. Rudy Giuliani is pro-choice. So is Hillary Clinton. According to the pro-life view, both of them are willing to tolerate serious evil, and that is immoral. However, even given the false premise that abortion is the only morally relevant issue, it simply doesn't follow that pro-life voters ought to stay home or vote for a third party if those two candidates receive their party nominations.
Even if abortion is the only issue under consideration, Hillary Clinton is far worse from a pro-life perspective than Rudy Giuliani is. When he was mayor of a very liberal city, he did virtually nothing to increase women's rights to have abortions, and the abortion rate went down. Some of that may have been just part of a national trend going on at the same time, but it doesn't seem as if he cared enough about the issue to promote abortion rights, never mind to expand them. Rather, he seems to have been expressing a pro-choice view mainly because he's not too motivated by pro-life concerns and not because he holds Hillary Clinton's view that the right to abortion is so inviolable that we should never restrict it under any circumstances.
He seems open to letting states decide, as is his general view on many issues. He worked in the Reagan Justice Department, which suggest some kind of judicial conservatism, and he has gone on record supporting judicial nominees like Roberts and Alito, as opposed to those like Kennedy, O'Connor, or other Republican appointees who have safeguarded Roe v. Wade. Even if the pro-life voter can't trust how faithful he'd be to that, he obviously isn't so dedicated to the pro-choice view that he'll let it affect anything else he does as if it's one of the most important rights one might poseess, which is exactly what Hillary Clinton would do.