David Campbell and Robert Putnam say that Romney's Mormon Challenge May Not Be Insurmountable:
It's clear, then, that whereas evangelicals present a problem for Mr. Romney as he competes in heavily evangelical primary states like Iowa and South Carolina, his Mormonism would be unlikely to hurt him if he survives and wins the Republican nomination. Neither secular nor minority voters are prone to vote for Republicans anyway, and evangelicals are equally unlikely to cast a ballot for a Democrat. Of course, evangelicals may hurt a Romney candidacy by staying home on Election Day 2012, but their strong opposition to President Obama and their past high levels of turnout suggest that they will take to the polls to try to oust the incumbent. Meanwhile, other churchgoing Americans—especially white Catholics and mainline Protestants—appear unconcerned with Mr. Romney's religion.
Thus our data suggest that the key question isn't whether a Mormon can be elected president, but whether a Mormon can win the GOP nomination. Should Mr. Romney clear that hurdle, the evidence suggests that the general election would not hinge on his religion.