Ed Kilgore asks a good question: Why are evangelicals 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) ... 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.
As Kilgore notes, there's nothing obvious or inevitable about this breakdown:
Catholic anti-abortion views, after all, are undergirded by a long series of increasingly emphatic papal encyclicals; a natural law and bioethics tradition stretching back all the way to Aristotle; an overall theological position making church teachings on matters of faith and morals binding on believers; a relatively low level of tolerance for individual dissent; and a teaching and disciplinary system that can be (and in some parts of the country, is being) deployed to influence the views and behavior--personal and political--of the laity.
Not one of these is a significant factor for Sola Scriptura Protestants. And unlike other moral issues ranging from gay and lesbian rights to divorce to adultery, the belief in scriptural inerrancy common among evangelicals doesn't really explain the vast gap between evangelicals and their mainline brethren on abortion. I've yet to read or hear a purely scriptural justification for banning abortions that doesn't ultimately come down to circular reasoning based on the condemnations of homicide from the Decalogue onward.
Evangelical hard-line views on abortion are not a matter of an unbroken tradition. In 1971, before Roe v. Wade, when nearly all states maintained abortion bans, the Southern Baptist Convention passed a resolution calling for abortion laws that would recognize exceptions not only in cases of rape and incest, but where the "emotional, mental and physical health of the mother" might be endangered. Needless to say, that would be considered a radically liberal position among evangelicals today.
Kilgore proposes a couple of possible explanations for this phenomenon - perhaps evangelicals are more "radically alienated from contemporary American culture" than Catholics and thus more likely to read America's abortion laws as "a symbol of a depraved society"; or perhaps they are more likely than Catholics to reject the arguments about "church-state separation and protection of individual conscience" that tend to undergird the case for a laissez-faire abortion policy. But I suspect that there's something simpler going on as well: Namely, that describing oneself as an "evangelical" tends to be a proxy for religious intensity in a way that describing oneself as a Catholic isn't. Many evangelical churches subsist within mainline denominations, attracting a self-selected pool of the denomination's most devout churchgoers; many others, especially in the megachurch sector, rely heavily on spiritual seekers looking for a more intense experience than their mainline upbringing (or Catholic upbringing, more often) provided. If you're a member of an evangelical church, chances are your congregation demands more from you - and you demand more from your congregation - than even the minority of Catholics who fulfill their Sunday obligation every week, let alone the lukewarm, once-a-month variety. And if you're born and raised evangelical, you're getting a very different experience of religion than the typical cradle Catholic, since evangelical youth ministries tend to emphasize the necessity for personal conversion - of making an active choice for Jesus, and being "born again" - much more heavily than your average Catholic confirmation class. American Evangelicalism is thus at a deep level a religion of converts and enthusiasts in a way that American Catholicism - which of course includes its share of converts and enthusiasts - simply isn't. And it's hardly surprising that this difference would manifest itself in polling on abortion and related matters, since as a general rule (with, of course, myriad exceptions), the more seriously a given Christian takes their faith, the more likely they are to come around to some variant on the pro-life position. I suspect that if you polled Catholics based on their religious intensity, the most intense RCs would resemble the average evangelical on life issues: It's just that the average evangelical, by virtue of being an evangelical, is more religiously intense than the average Catholic.