Mitt Romney will not be the Republican presidential nominee for two related reasons.

1. Conservative Christians do not regard Mormons as Christians, instead terming them a "cult."

2. Conservative Christians believe the United States to be a Christian Nation, needing Christian leadership.

Therefore, since Mitt Romney is not Christian, he cannot be allowed to lead a Christian nation.

If, contrary to my prediction Romney does become the Republican nominee and faces off against Hillary, then Conservatives probably would vote for Romney. He may not be Christian, in their thinking, but is not seen as Anti-Christian in the same way Hillary is. (more below)

As a scholar, I also think Mormonism is not part of Christianity; I regard it as a New Religion historically related to Christianity. Thoughts here. Regarding America as a Christian nation see here.