For me, the political issue has nothing to do with whether Mormons are or are not Christians. The Presidency is not a religious office, though it clearly can play a role in articulating our civil religion. And while our civil religion surely has “Judeo-Christian” (or, more broadly, Abrahamic) elements, it is certainly possible even for non-Christians more or less sincerely to evoke these elements as part of our cultural heritage (the so-called “ceremonial deism” that even some of the more fastidious members of the Supreme Court can countenance).
At least as important as the President’s role as articulator of our civil religion is his or her role as a constitutional chief executive. Here the question is the degree to which a candidate properly understands the constitutional limits of the office and of the federal government. This understanding has nothing to do with a person’s faith or lack thereof, It’s possible to imagine a sincere Christian who is an adherent of the “living Constitution” school of jurisprudence and a skeptic who’s a strict constructionist.
Then there’s policy. We might get general principles from Scripture, natural law, or common grace, but how to implement or instantiate those general principles in public policy is matter for prudence. Reasonable people can disagree about many of the issues, and even where the teaching of a religious tradition seems clear, our practical political choices rarely offer unmixed blessings or curses.
Finally, there’s character. There are scoundrels in the pews and decent people of integrity who don’t go to church. In order to be confident of my judgment on the three aforementioned grounds, I also have to be confident that the candidate is—and will (within the limits of human sinfulness) continue to be—who he or she claims to be.
If Mitt Romney satisfies me on all these grounds better than any of the other plausible options, I’ll vote for him. His Mormonism would matter to me no more than would Joe Lieberman’s Judaism, Rick Santorum’s Catholicism, or John Ashcroft’s charismatic Christianity. (Yes, I know Ashcroft isn’t running for President, but I couldn’t think of another politically prominent charismatic off the top of my head.)