Unless you happen to believe that discussion of religion is speech without protection from the First Amendment.
The question isn't "Can Mitt Romney be president?", because he was born here and is at least 35 years of age. He meets the constitution's test.
But politics imposes a different kind of test than asking "Can he?"
The political question is "Should he be president?"
Some people won't vote for Mitt Romney because he's a flip-flopper on some important issues. Some won't vote for him because he supported TARP. Others won't vote for him because he's a Mormon. Still others won't because of RomneyCare in Massachusetts.
But probably the largest group which won't be voting for Romney will be Democrats, simply because Romney is a Republican. But you won't hear the media's C-students bemoaning that as a stupid reason not to vote for someone.
Ken Connor attempts to clarify the matter here:
"[T]he question remains, 'Are Mormons Christian?' Since Mormons assert that they are, isn't this a fair subject of public debate? Since religious truth claims have eternal consequences, isn't it in the public interest to examine the merits of those claims? The election of a Mormon president will likely do more than any other single event to mainstream Mormonism into American life, with all of the consequences attendant thereto. That being the case, why should the claims of that religion be any more immune from scrutiny than those of any other religion?
"America will be a poorer country when we reach the point where discussions about religion and our eternal destiny can no longer be part of the public dialogue."