The Constitution, Article VI, Section 3
No religious test shall ever be required as a qualification to any office or public trust under the United States.
I agree with Eugene Volokh that the No Religious Test clause doesn’t mean that no official making an appointment and no voter considering whom to vote for may consider the religious affiliation of potential appointees or candidates. In historical context, it was clearly designed to forbid “test oaths,” such as the Oath Against Transubstantiation, which intended to exclude people holdfing particular religious views (e.g., Catholics) from office by requiring all office-holders to take an oath that no member of the sect to be excluded could conscientiously take.
But I also agree with him that using someone’s religion as a campaign issue is un-American. Since Eugene tends to favor the Republicans, I’m especially gratified to see him calling a foul on his own side.
If the President of the United States, a Texas Republican, has any objections to the latest antics of the Texas Republican Party, he’s keeping quiet about them.
I am an atheist, and someone who has described himself as a 'tolerant atheist,' but I can't go along with your statement that 'using someone's religion as a campaign issue is unAmerican.' (Even though I know that by saying this I preclude myself from any political office at present — not that I could be elected for other reasons.)
I do not see why, for example, a person's attachment to the Unification Church, to Scientology, to Farrakhanism, or to various dominionist/reconstructionist radical forms of Christianity are not very important towards understanding how someone will act in office. (Especially if the person has hidden such attachments.)
Hopefully, if atheism were used as an issue, the person involved would have a chance to explain what this means, and might turn people away from this prejudice. For all I know, members of the religions I mentioned could do the same. But denying their relevance is absurd — which is not to excuse misrepresenting them, as the Texas ("the US is a Christian Nation") Republicans did with Franks' atheism.