Because he is Mormon voters will not support Romney.

Reasons to agree:

1. Some religious people try to demonize those of other religions. To say that you agree with them on anything is to admit that they are people, and that they are capable of rational thought. It is better to say that everything they do is bad.
2. Some religious people look at religion is a competition, and any time someone does well from another religion or attains a place of power they think that it hurts them and their religion.
3. Some people don't vote for a candidate as an individual they vote for the candidate based on which group he or she fits into. They vote for someone who is from the same group that they are, weather it is race, religion, economic background, career, etcetera.
4. People who think that a certain religion is stupid will naturally think those that believe that religion to be stupid. They will question their judgment on other matters.
5. “I spent this weekend with an evangelical conservative Republican group and everyone said, ‘Well it doesn’t bother me, but my friends.’

Reasons to disagree:

1. "Recent polls have found that around 40 percent of Americans say they would not vote for a Mormon for president -- though nearly as many said they would have reservations voting for a Catholic in 1960, the year John F. Kennedy won the White House."
2. Most theological questions are irrelevant to the presidency.
3. Mormon's aren't the only weird religion. Every religion believes weird things, and has some weirdo’s and skeletons in their closets.
4. Most people don’t look at the religion of someone when they decide weather or not to vote for a candidate.
5. It is not in our countries self interest to discriminate based on religion, when hiring in business or government. People will eventually do what is in their own self interest.
6. Some people might vote for Mitt, just because the novelty of having a Mormon president, or to embrace tolerance.
7. Main stream religious leaders] don't have problems with him.
8. "Yes, Gov. Romney is a Mormon. We are not. According to the liberal media, this is an unbridgeable gap, and evangelicals will never turn out to support a faithful Mormon like Governor Romney. As usual, the media have it wrong. And they root their error (as usual) in a fundamental misunderstanding about American evangelicals—seeing us as ignorant and intolerant simpletons who are incapable of making sophisticated political value judgments." http://www.evangelicalsformitt.org/why.php