I have gotten this question a few times, and I thought I'd give it a run here and hope it proves helpful.
Mitt Romney is a Mormon. He's the Republican nominee for President of the United States. Most evangelical believers lean farther right than they do left, though they would say (and I would agree) that neither party is particularly helpful in some major areas of life and government.
So the question comes: can an evangelical Christian vote for a Mormon?
First, let me say what a privilege we have that we can vote at all. That's not true everywhere in the world. It wasn't even true of every person who was a citizen in this country just a mere century ago or so. Don't take it for granted.
Second, whether you vote for Romney or Obama is between you and Christ. Christians have the responsibility to vote their conscience, having prayed through the decision and sought God's wisdom on who would be best for the country. This is not my political statement on telling you how to vote. Vote as Jesus leads you, nothing more or less.
Third, be clear as to what Mormonism is and isn't. It is a religion on the landscape of the world that produces nice, moral, upstanding citizens. It is NOT a Christian religion in any normal sense of the word. Christianity hinges on the claims to deity of Jesus Christ. If He's not God, He has no way of taking the blow of God's wrath on the cross in a redemptive way. As well, if He's not God, He has no guarantee of His victory over death by His resurrection.
Mormons use terms like Son of God in ways that Christians are familiar with but do not enforce the same meaning. Their understanding of Son of God would be more akin to Son of Trent, as in offspring. Son of God in normal Christian parlance means the second person of the Trinity, the person who is fully God, coequal and coeternal with the Father.
So please don't make the mistake of assuming that common language carries common meaning. "That's cold" means one thing among teenagers, another among scientists, and another among adults enjoying beers in the backyard. It's true in theology too.
Fourth, I point you to an instance in history that my friend Tommy Kidd has pointed out on his blog (go read it, it's a great article) where a group of conservative Christians, the Danbury Baptists, supported someone for President who was not even close to their beliefs - even denying some of the basic ones, like the deity of Christ. In 1800, Thomas Jefferson got the support of the Danbury Baptists because they knew he was a believer in religious liberty, even though they shared nothing in common theologically. The Danbury Baptists knew that politics requires at times partnership with those who do not hold your theological convictions in order to promote the good of society. The same lesson still applies.
So can Christians vote for a Mormon for President?
My answer is maybe. The question is whether or not his presidency will promote the good of society. And that hangs on his character primarily, along with his leadership capacity, pertinent experience, and his courage to do the right thing in the tough moment.
But that's just me thinking thoughts...