The Pew Forum on Religion and Public Life recently published a study comparing the religious makeup of the US Congress compared to that of the American public.
The operative question seems to be, "Does Congress look like America in terms of religious tradition?" Should it?
The short answer is that some larger groups (like Protestants, Catholics, and Jews) are somewhat over-represented, while some smaller groups are under-represented. Most striking is the under-representation of "Unaffiliated" folks.
So it seems that Americans in general like their politicians to be mainstream, from a readily identifiable tradition, and at least somewhat religious. Would you vote for someone who seemed religiously exotic?