While I personally find it hilarious that many Southern Baptists think their church legitimate and LDS is a cult, in the hypothetical Obama v Romney general, they are coming out to vote against Obama, not to vote for Romney.
I doubt it. I think a lot of Christians who are against the president would simply sit this one out. They wouldn't vote for Romney, and they won't vote for Obama. Their only options are to either go with a third party candidate, or to not vote at all, and I think many will choose the latter.
I'm a Christian, non denominational, and if this is the match up I'll be voting for Obama (based on political reasons, not religious ones). I have no problem voting for a Mormon, just as I have no problem voting for an African American, Hispanic, etc. Romney is a nut on the same level as Palin, he only hides it a little better.