Prior to Mitt Romney's withdrawal, there were five exit polls in states with high percentages of evangelicals that asked how much a candidate's religious beliefs mattered: Alabama, Georgia, Oklahoma, South Carolina, and Tennessee. Mike Huckabee won Alabama, Georgia, and Tennessee, and came in second to John McCain in Oklahoma and South Carolina. Romney came in third in all of them except South Carolina, where he finished fourth behind Fred Thompson. The question is: To what extent did anti-Mormonism play a role in his poor showing in these states?
Pending a multivariate regression analysis by some competent social scientist, my eyeball analysis suggests that it played a small but significant role. In all of the states in question, Romney polled anywhere from one-half to one-third as well among those for whom a candidate's religious beliefs mattered a "great deal" as he did among those who answered the question: "somewhat," "not much," or "not at all." The idea here is that most of the voters in the "great deal" category were evangelicals, and that many of them were simply unprepared to vote for a Mormon.
To be sure, it could be argued that these voters were simply disproportionately attracted to their co-religionist, Mike Huckabee. And, indeed, Huckabee did disproportionately well among them. However, if it were simply a question of attraction to Huckabee, then the effect on McCain's vote should have been the same as the effect on Romney's--and it wasn't. McCain did about two-thirds as well among those for whom a candidate's religious beliefs mattered a great deal as he did among those who for whom it mattered less.
How much difference this made to the final tallies is hard to say. Romney finished close enough to the top in Georgia to suggest strongly that, but for anti-Mormon prejudice, he would have won that state. Elsewhere, maybe not. But there's no reason to suppose that, just because Romney's religion faded as a media story after the first of the year, it faded as a factor in the voting behavior of a significant segment of the Republican rank and file.