The Pew Forum‘s analysis of the exit polls by religious affiliation has an interesting takeaway. Mitt Romney was expected to do well among his fellow Mormons: Mitt Romney was a lay leader in the Church of Latter Day Saints, serving as bishop of the ward from 1981 to 1986, and indeed 78% of Mormons supported Romney.
However, Mormons are typically very conservative voters. For example, George W. Bush earned the support of 80% of Mormons — two percent more support than Romney earned.
Some Mormons were quite vocal in their opposition to their coreligionist. See for example this videos from the Mormons for Obama website.