Religion in the USA matters. In fact, Americans would prefer a homosexual to an atheist (or at least that is what they say when answering one of those rushed surveys). According to that same survey, 94% would vote for a Catholic.
Note that this does not mean that for 94%, a candidate's religion doesn't matter, just that they would consider voting for a candidate of said religion.
Oddly, the survey doesn't ask if Americans would be willing to vote for a Protestant.
Anyway, Obama is Protestant, Biden is Catholic. Romney is Mormon, his running mate is Paul Ryan is Catholic.