...suppose a politically savvy Rip Van Winkle in say, 1965, perceiving that a movement to legalize abortion was gaining strength in the country, were asked, "Which of the two major political parties will eventually identify with that movement?" What would he answer? I think he would mull it over in his head for awhile and then say: "the Republicans, probably."
Why? "Well, in the first place, [abortion] fits pretty well into the Republicans' private-property philosophy. 'Let's keep government out of a woman's most personal property.' Secondly, consider the demographics. The Republicans draw heavily from the upper-middle class WASPs, where the drive for population control has always come from. Abortion fits very well into the old eugenics mythology -- the belief that you can improve the health of the 'race' by limiting the breeding of 'undesirables.' You can still hear echoes of that in the conversations of bicoastal Republicans. It wouldn't surprise me at all if the Republican Party came out with a plank saying 'We support abortion, in certain cases, for the nation's overall health and well-being.' Finally, consider the Republicans' emphasis on the need for law and order and their conservative approach to welfare. The Republicans may not say this out loud but it slots right into their conservative ideology: abortion is good because, by holding down illegitimate births, it will cut down on crime and welfare costs."
What about the Democrats? "Well," Rip would say, "let's start again with demographics. Consider the heavy concentration of Roman Catholics in the Democratic Party. The Church hierarchy would go bananas if any prominent Catholic Democrat -- or any Democrat at all --came out in favor of abortion. The Church has consistently held that abortion is one of the gravest moral offenses because it involves the direct killing of an innocent human being. No way is a Catholic Democrat, or any Democrat who wants Catholic support (and what Democrat doesn't?), going to support abortion. It might even be smart politics for the Democrats to pick a fight with the Republicans on the abortion issue. Democrats like to boast that they protect the weak and the vulnerable. You remember Vice President Hubert Humphrey's characterization of his party as the advocate of those 'who are in the dawn of life; those who are in the twilight of life, the elderly; and those who are in the shadows of life, the sick, the needy and the handicapped.' All they have to do is insert 'unborn children' into that list and they can beat up Republicans every time on the abortion issue. I can hear them now: 'Let the Republicans pick on the weak and vulnerable, killing children in the womb to cut welfare costs. We Democrats are the party of compassion, the party that sticks up for the little guy, including the littlest guy of all, the child in the womb [Applause].'"
-George McKenna, Criss-Cross: Democrats, Republicans, and Abortion (PDF)
McKenna also puts forward the idea that intra-Catholic conflicts are a driving force in abortion politics. Catholics climbing the social ladder shed their admirable tribal Catholicism for a decadent tribal liberalism, then looked askance at those who remained in the old tribe.
He also notes the academic structures feeding compromised counsel to weak-willed clerics in the episcopacy:
Anyone who thinks that the bishops operate independently, handing down decrees and getting those below to obey, has it almost exactly backwards. The bishops’ pronouncements well up from currents of thought circulating among people below them, in some cases from those far below them. Not from the pews, though. From Catholic seminaries, from Catholic journals and theological associations, from philosophy and theology departments in Catholic universities, and, most immediately, from the staffers who serve the National Conference of Catholic Bishops. The overwhelming majority of those occupying these seats of influence are Democrats, and some are Democratic activists. For them, any move toward condemning the Democrats’ position on abortion only helps the Republicans, and helping the Republicans only helps to inaugurate or perpetuate social policies that are, in the final measure, un-Christian. Therefore, to single out the Democrats’ abortion plank for condemnation is to side with the forces opposed to the Church’s program of peace and justice. Objectively speaking, as the Marxists used to say, it is anti-Catholic.