I'm a Catholic, but I have a ton of Evangelical friends. My impression is that a great many Evangelicals would draw the line at voting for a Mormon for president. That pretty much eliminates Romney right there, because without that vote he can't possibly win.
The official name of the Mormons is the Church of Jesus Christ of the Latter Day Saints. Up until recently, the emphasis was pretty much on Latter Day Saints. But in recent years, Mormon leaders have been putting in a claim to be Christians, so there is more of an emphasis on the first part of the name.
For those interested in some of the theological difficulties, there's a useful article in First Things from a few years ago, written by Father Neuhaus, "Is Mormonism Christian?"
Here's the article. If you search the website for "Neuhaus Mormon" you'll find some of the letters written in reply.
http://www.firstthings.com/ftissues/ft0003/public.html#mormonism