But that is a theological argument. I, of course, agree that the Trinity is a core belief of Christianity, but Mormons make great neighbors because they are generally very ethical people. There's no reason why one would not make a good president.
I'd vote for a Mormon before I'd vote for a satanic liberal any day.
I agree that generally Mormons make great neighbors (for a variety of reasons). Could a Mormon be a good president? I think it's possible.
But I think larger issues are key than simply "could X be a good president?" I think timing is a prominent concern. (In an era where National Security is challenged more than ever, do we want someone who prays to a God distinct from the God of Abraham, Isaac and Jacob as our leader?).
I haven't thought this fully through yet, but we need to ask if having an LDS president will turn out the same public relations way as having Utah host the Olympics?
Functionally, did Utah do a great job of hosting the Olympics? (Yes) Was it used to further spread LDS good will and further its proselytizing efforts? (Yes)
Whether the above is a strong enough reason to vote against someone is another issue. At least, it's an issue to put on the table for discussion.