Anybody who can look at Mormonism and believe in it is not fit to lead a country. Some might say that for Christianity as well, but at least our holy book is the same as it was thousands of years ago. Since the 1850's, they've had to edit theirs 4000 times. It was said to be a perfect book but it has spelling and grammar issues.
Joseph Smith's revelation included polygamy but they have backed away from it. They believe their 'perfect book' trumps the Bible, which would be fine if the Bible actually was corrupted and the Book of Mormon didn't have all those mistakes... then maybe somebody could believe it and be rational.
Also, the Book of Mormon speaks of many cities in the Americas that were never there, or at least, that there is no archaelogical evidence to support it. The Bible has had over 25,000 archaelogical discoveries which prove that it is historically accurate. 0 for the Book of Mormon. But they say the Bible has been corrupted.
A long time ago, somebody tried to say the Bible was corrupted and created his own book of what was the true religion. His name was Mohammed.
So, at it's most basic level, we are choosing between which brand of Islam we want to allow to rule us.