JimBob is right, it's in how you define "denomination." If you break down denomination to mean "type of Christian" with everything from Baptist, Methodist, Church of God, Catholic and so on, Catholics will be the largest group.Nope, a simple search reveals that Protestants are the largest Christian denomination in the U.S. In fact, there are about twice as many Protestants as Catholics.
However, I don't agree that Evangelicals are the only ones who actually believe, go to church, etc. I'm an Episcopalian and the people in my congregation, actually believe, go to church and act on their beliefs. They are also mainly liberal. We have women bishops, welcome gays, and are probably pro-choice, but I don't know for sure because it's never come up in church. For that I am truly grateful.
Abortion is just one issue among hundreds. I don't understand why it gets quite so much violent reaction on both sides or why it would be assumed that all Christians were pro-life or all non-religious were pro-choice.