Ed Kilgore’s interesting post over at Progressive Revival about evangelicals being more pro-life than Catholics has stirred some interesting debate.
Kilgore is puzzled but offers a few theories:

1) Evangelicals are “are radically alienated from contemporary American culture, and view legalized abortion (along with premarital sex, open gay/lesbian lifestyles, and TV/Hollywood “trash culture”) as a symbol of a depraved society.”
2) “framing” of the abortion issue–particular its treament as fundamentally a matter of the reproductive rights of women, or of personal privacy–that underlies the pro-choice argument is simply uncompelling to many white evangelicals…. American evangelicals have become strongly averse to the libertarian traditions of church-state separation and protection of individual conscience that once was a central feature of their own belief system.

Ross Douthat at the Atlantic Monthly responded by suggesting that people who describe themselves as evangelical are simply more intense in their religous feelings than are Catholics, a broad label that includes mass-attenders and nominal Catholics.
Some evidence for Douthat’s theory can be found in the Pew Religion Landscape survey: 58% of Catholics who attend mass weekly — the most intense Catholics — want abortion to be illegal all or most of the time. That compares to 61% for evangelicals generally and 73% for evangelicals who attend weekkly. So intense Catholics have about the same views as generic evangeliclals.
I have three more theories to throw on the pile:
1) Homilies vs. Sermons — While the Catholic Church has been articulate in writing about its opposition to abortion, most Catholics don’t sit around reading encyclicals. The question is: does their parish priest preach about the evils of abortion as persuasively, emphatically and frequently as as a evangelical minister? Evangelicalism in general emphasizes the sermon more than Catholicism does, so I’m guessing that the typical evangelical simply hears the case against abortion more frequently and persuasively than does the typical Catholic..
2) The Evangelical Bundle — Though this is eroding, my sense is that being an evangelical meant you were signing on to a particular bundle of conservative ideas. You were getting the car, fully loaded. Being an evangelical meant (at various points) opposing the Panama Canal treaty, gay marriage, Democrats and abortion. To truly feel fully part of the church community, you were better off signing on for the whole package. Because the Catholic Church cannot be considered uniformly conservative or liberal, Catholics felt like they could pick and choose more.
3) Contraception — This theory has even less basis in fact than the first two but… The Catholic Church’s position against birth control was so rapidly and pervasively rejected by rank and file Catholics that it undermined the Church’s authority on “sexual” issues in general. Or, to stated it more conservatively, if you don’t care about losing the potential of a life because of a condom, why should we be surprised that you don’t care about losing a one-week-old life through an abortion?