Ed Kilgore has a fascinating essay at Beliefnet about the fact that Evangelicals are foar more pro-Life than Catholics:
There are variable measurements of this phenomenon, but no real doubt about the basics. A September 2007 Pew survey showed white evangelical Protestants agreeing that abortion should be illegal in all or most cases by a 65-31 magin; Catholics favored keeping abortion legal in all or most cases by a 51-44 margin (with no appreciable difference between Hispanic and non-Hispanic Catholics). On a related issue that helps measure the intensity of anti-abortion views, the same poll showed white evangelicals opposing embryonic stem cell research by 57-31, while white non-Hispanic Catholics favored it by 59-32.
Moreover, the evangelical-Catholic gap on abortion looks likely to increase in the future. An April 2004 Pew survey providing generational breakdowns showed that white evangelicals under 35 favored abortion restrictions by more than a two-to-one margin (71% among those under 25), while those over 65 actually (if narrowly) opposed more restrictions. The generational trend lines among white Catholics moved in exactly the opposite direction.
Ed then explores why this should be the case, given the consistent and forceful teachings on abortion by the Catholic hierarchy. He offers some conjectures, but even he is not very satisfied with his answers:
so whence cometh today's white evangelical anti-abortion ferver? One theory is that these folk 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. This is undoubtedly the view of some well-known evangelical leaders like James Dobson, who often indulges himself in Nazi analogies for the "Holocaust" of abortion. But objective measurements of evangelical cultural alienation are generally ambivalent, and they are famously enthusiastic about adopting contemporary culture in their own liturgical and missionary practices.
Another theory, for which I can offer little other than plausible conjecture, is that the "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. Aside from the strongly anti-feminist bias of much of contemporary evangelical teaching, 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. And perhaps an inability to even hear the pro-choice case has reinforced the impact of such secular phenomena as widely available sonogram images of fetal development.
The bottom line is that I don't know, and I'm not sure anyone knows, if Barack Obama or any other pro-choice, pro-gay rights, pro-feminist politician or party can make significant inroads into the white evangelical vote by minor tweaks in abortion policies or how they are presented. Evangelicals, of course, care about other issues like the war in Iraq, the economy, the environment, and corruption in Washington, that could incline them towards a vote for a Democratic presidential or congresstional candidate. And that's why (along with chronic disappointment with GOP promises to "deliver" on cultural issues like abortion) so many evangelical leaders like Rick Warren are expressing an openness to two-party competition.
Read it all here.
I doubt that the answer lies in religious doctrine. Instead, I think the answer lies more squarely in culture. Abortion is but one of many issues in which many American Roman Catholics quietly disagree with church teachings.