Your hypothetical choice is just that, hypothetical. It would depend on the candidates and the circumstances.
For instance, President Bush when he first ran said that he was against abortion except in cases of rape, incest, or life of the mother. I don't agree with the first two exceptions, but frankly I couldn't conceive of any practical circumstances where it would matter, because I doubted that it would have any practical influence on his policies as president.
And I don't think it has. Bush has been consistently pro-life, and has done a great deal to further the cause. The only thing he actually did that I disagreed with was to agree to fund current lines of fetal stem cells for research, which I think was wrong. But that's far outweighed by his positive actions.
If I looked at someone who might conceivably, as president, succeed in outlawing all abortions past the first trimester, then, sure, I would vote for such a person against an all out abortionist. It's not like committing a lesser evil; it's an improvement of the current situation.
Or someone might say that, but I might evaluate it as meaningless. This person would do nothing to improve the situation. In that case, depending on who the Democrat was, I just might not vote, because the last thing we need is to increase the number of pro-abortion Republicans and end up with two pro-abortion parties instead of one, or maybe one and a half.
Thank you for the response. I am in no way trying to open a debate on the evils of abortion but rather using it as an example of some of the choices we are faced with politically. There are times when we must support someone who is not 100% ideologically aligned with our individual values because the choices simply aren't there. In those circumstances we make individual choices based on what we think are advancing our cause. I believe that was the point Sen. Bob Casey was trying to make.