In the book, Discourses of Brigham Young, an edited collection of President Young's sermons from the Journal of Discourses, we read:
"How long will it be before the words of the prophet Joseph will be fulfilled? He said if the Constitution of the United States were saved at all it must be done by this people. It will not be many years before these words come to pass." 12:204.
*update* While Romney claims that his presidential decisions will not be influenced by leaders of the Mormon church, everyone who knows anything about Mormonism knows that is not true. Temple endowed Mormons covenant to do everything in their power to build up the kingdom of god on earth. His positions on certain political issues are most certainly influenced by the church. The laws he will try to pass will all most certainly reflect LDS rules and standards.
Remember the LDS church has always wanted power. What more power can they get than one of their own as president of the most powerful nation in the world?