I’m not sure that I agree that Evangelicals have marginalized themselves politically, as this Evangelical (whom I respect) appears to believe.
But it’s striking that the 2012 major party presidential tickets are, respectively, Barack Obama (liberal Protestant) and Joe Biden (Roman Catholic), on the one hand, and, on the other, Mitt Romney (Mormon) and Paul Ryan (Catholic), with nary an Evangelical in sight, and that the current Supreme Court lacks even a single Protestant member of any kind:
Ruth Bader Ginsburg, Stephen Breyer, and Elena Kagan are Jewish. Antonin Scalia, Anthony Kennedy, Clarence Thomas, Samuel Alito, Sonia Sotomayor, and Chief Justice John Roberts are all Catholics.
Some years ago, with such figures as Jerry Falwell and the “Silent Majority,” American Evangelicals were really beginning to flex their political muscle. It’s a bit obscure, now, where all that currently stands.