Wow, got this from a very influential evangelical friend:

I don’t think most evangelicals are afraid that a President Romney will impose his esoteric Mormon morality on the rest of us. We’re not really worried he’ll try to ban caffeine (though Huckabee might), or hand out tax breaks for special underwear.
We’re afraid that nominating a Mormon will legitimize a cult.
Maybe there’s something Romney can say to assuage that fear. He didn’t say it today. It was a waste of time to reprise Kennedy’s speech.
Which makes his faith a legitimate concern for GOP primary voters. Why? Because Mitt will depress turnout. Many voters who might otherwise pull the GOP lever will stay home debating whether it’s worse to mainstream Mormonism forever or let Nurse Ratched take over the asylum for four years. Therefore, faith aside, he’s the wrong choice because he’s less likely to win.
It doesn’t matter how convincingly Romney claims he’ll compartmentalize his faith. Nor does it matter if, say, Romney makes the Huckster his veep. In fact, Romney taints the ticket whether he’s the presidential OR the vice presidential nominee.

This is what a lot of evangelicals really think but are afraid to say publicly.