I guess it depends on where you draw the line on fundamentalism. I live in the Bible Belt and I'd argue that, at least, the Southern Baptists should be seen as a Fundamentalist organization. That locks down the majority of the people in the SE-quadrant of the continental US.

Also depending on how you draw your lines, the Catholics aren't even remotely close to #1 status. About 24% of Americans are Catholic while about 51% are Protestants. I'll agree that it's a little iffy, but I'm willing to count the Protestants as a singular group.

Perhaps I'm just jaded because I live in the land of "lets turn the US into a Christian theocracy".