You make an interesting point about the religious angle. But heavily Baptist Oklahoma elected Dewey Bartlett (1972) and Don Nickles (1980, 1986, 1992, & 1998), who are Roman Catholic.
As much as some ignorant Baptists may dislike Catholics, I think ignorant Baptists dislike Mormons a lot more. Mormons probably face more prejuidice than any other Christian denomination in America (actually, one of the bones of contention for Mormons is that many Christians do not consider Mormons to be Christians). If Istook runs, he will need a widescale voter education campaign so that conservative voters don't take any biases into the voting booth.
I think religious voters need to concentrate on the candidate's issue positions, not his religion. It's a shame, for example, that Kerry got over 40% of the Catholic vote in spite of his pro-abortion, anti-family views; I'm sure a lot of these votes came from Catholics who wanted to "vote for one of their own." I wish communities of faith did more to educate believers regarding how deeds are more important than religious labels.
BTW, I think Baptists for the most part are quite accepting of Catholics; I'm a Catholic, and have Baptists have never been anything but warm and cordial to me, and my (Catholic) aunt teaches at a Baptist university and lives in a predominantly Baptist community and has never had any trouble (except a few polite smiles when she asked at a get-together if they had any beer).