As a pro-life Republican, maybe I can shed some light on the problem. The Republican party serves (poorly) three different conservative constituencies. It is an awkward alliance, with each group gaining ascendancy at different times.
First, the libertarian wing of the party doesn't care much about abortion. They are more interested in avoiding the liberal "nanny state," in which the state controls the minutia of everyday life. That would include environmental land-use laws that prevent people from buiding homes on their own land, laws dictating what kind of grocery bags you are allowed to use, and laws dictating where your coffee must come from.
Secondly, you have the corporate interests, or the "country club" Republicans. These are the people that are represented by your Karl Roves and Dick Cheneys. They are the first in line for their corporate welfare checks. Frankly, many conservatives are fed up with them, since they give all of us a bad name. Again, they are not very engaged in the abortion question.
Finally, you have the social conservatives such as myself. These are the ones that oppose gay marriage, favor strong law enforcement, and are strongly pro-life. This group may have some progressive ideas about helping the poor and opposing war, but their pro-life convictions make it impossible to vote for Democrats on the national level.
Maybe this analogy might help: Imagine a candidate who is a staunch feminist, pro-choice all the way. This candidate is a true environmentalist, and makes Marx look like a captain of industry. Now, imagine that this candidate is an unrepentant racist. Could you vote for her? Probably not. I believe that the current practice of abortion is such a heinous act that it overshadows most other considerations. It may even cost Giuliani the nomination.
Now, a quick outline of why I oppose abortion on demand. First, a fetus is human life. It is living tissue, and in all ways it is genetically human. Is it viable, able to live without the mother's support? No, but then again, all newborn infants are incable of living on their own. A human fetus is a potential human life, valuable for that potential. In addition, there is no moral equivalence between an innocent child and an adult murderer, so don't give me any "but what about capital punishment" garbage.
Secondly, being pro-life does not mean that I oppose a woman's right to choose whether or not to bear a child. I just think the decision should be made BEFORE the sex act. I believe that contraception is perfectly acceptable, and that men should always respect a woman's decision NOT to have sex. In fact, I can accept abortion in cases of rape and incest, where a woman may not have had a choice. The same goes for danger to the mother. However, only a total idiot would believe that sex was not designed to produce offspring, and should be respected as such. I believe that people must consider the consequences of their actions before they act, and must accept those consequences.
Yes, we know we're being taken advantage of by the country club Republicans. But here's where the difficulty lies: Do we vote for the candidates who merely pay lip service to our convictions, or the ones who heap abuse and contempt on us on a daily basis? I'm sorry, but people like Jon Stewart have made it impossible for me to EVER vote for a pro-choice Democrat.