That doesn't mean that voters are prohibited from taking a person's faith (or lack thereof) into account when deciding for whom they will vote. No law could stop them.
Past elections have been decided when some Catholics voted for a Catholic politician because of their shared religion and Protestants voted against a Catholic because they did not share that faith.
Now come two Mormons -- Mitt Romney and Jon Huntsman -- and two evangelical Christians -- Tim Pawlenty and Michele Bachmann. There is confusion and division within once nearly solid evangelical ranks over what to do.
Some evangelicals say they wouldn't vote for a Mormon for president, even though Romney and Huntsman seem, on the surface, to fit with many of the political viewpoints of the majority of politically conservative Christians on social issues such as abortion and same-sex "marriage" (though Huntsman favors "civil unions" and Romney has been on both sides of this issue, as well as abortion, more than once).
Does it really matter what faith a president or presidential candidate has, or should everyone, regardless of their religious background, focus on their competence to do the job? Shouldn't the question answer itself?
I would vote for a competent atheist who believed in issues I care about over the most conservative Christian or Orthodox Jew who lacks the experience, knowledge and vision to do a good job as president.
Religion can and has been used as a distraction to dupe voters. Jimmy Carter made "born again" mainstream during the 1976 presidential campaign and many evangelicals voted for him on the basis of his declared faith. Yet Carter later revealed himself to be a standard liberal Democrat in virtually every category that mattered, from abortion and civil unions, to the economy, to weakening America's defenses and image worldwide.
What about Barack Obama's self-declared Christian faith? He attended the Chicago church of Rev. Jeremiah Wright, whose sermons frequently condemned America and contained what some took to be racial slurs. The president's faith has not distinguished his positions on any issue that matters from that of a standard liberal Democratic secularist. If a candidate says faith is important, shouldn't that faith take the person on a different path than what someone of little or no faith would propose? If not, what difference does faith make and why should it be of concern to voters?