Ross wonders why evangelicals are more likely to be pro-life than Catholics, even though the Catholic Church is in many ways the one with the harder line on reproductive issues.
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.
As an empirical matter, I'm sure that last sentence is true, though logically I'm not quite sure why that should be the case--abortion isn't really much taken up by the Bible, and there's no particular reason that a lack of religious belief should cause one to set the emergence of personhood at birth.
On the larger question, I think Ross is on the right track, but I might state it slightly differently: evangelicalism is self-selecting in a way that Catholicism isn't. Catholicism is as often a proxy for ethnicity as it is for belief; I observe Lent not because I believe in the risen Christ, but because my ancestors have done so for a couple of thousand years. Not that I self-identify as Catholic, but I know a lot of people who think of themselves that way even though their main connection to the Church is watching the occasional Hail Mary pass.
Evangelicals who stop believing in God, or biblical literalism, don't continue to call themselves evangelicals. The religion itself encourages forum shopping. Lukewarm Catholics, on the other hand, tend to stay put.