Christians tend to "stick" together when the alternative is against their moral values. All Christians share many of the same moral values. It is their particular doctrines that are different. Which is why there are so many different denominations.
Bush is an "Evangelical Christian" which includes the teachings of fundamentalism and dispensationalism. Bush stood for "basic moral values" when he ran for office, which ALL Christians share. But if he were to impose his particular doctrines and beliefs (such as the Rapture), he would certainly meet with much opposition, which he has.
This country was founded by those whose theology was mainly the Calvinistic Reformed theology of the Puritans and the theology of the Anglican Church.
Catholics entered the country later during the late 19th century.
Christianity is the most divided religion there is. But when confronted with a common enemy, they stand together. This was the "genius" of the Bush campaign.