It seems to me that this is hard to discern from exit polls. First, the exit polls aren’t measuring why people voted one way or another, but which candidate they supported and which demographic groups they belong to, so the only thing we can know with any certainty is the level of support, or lack of support, from those groups assumed to be more likely to have reservations about Mormonism than others. For instance, in Georgia, 72% of evangelicals voted for someone other than Romney. Roughly the same held true in Illinois, and it appears that Romney did relatively better in Illinois overall than in Alabama because the evangelical bloc was a much smaller part of the electorate. Given that polling has usually shown that about one third of evangelicals and at least a quarter of all demographic groups would not vote for a Mormon, this large vote for non-Romney candidates among evangelicals could be related to anti-Mormonism, but it is impossible to isolate that factor when other explanations are available. Maybe the evangelicals who supported Huckabee identify with him, and those who supported McCain are drawn to the putative “front-runner” or they are impressed with his biography–who can say? You would be able to find out only if you tried to get some answers about their reasons for voting one way or another.
This is an inherently flawed way of assessing the existence of anti-Mormonism, and the only way to gauge it properly would be to ask voters rather rudely after they say they didn’t vote for Romney, “Hey, did you not vote for Romney because of his religion?” Answering a pollster who asked you that might be difficult for many people, who don’t want to appear prejudiced (even if their opposition to Mormonism is based in disagreement and not prejudice), so even those results might be misleading. As it is, we have no such information. It certainly seems possible that among the 60-odd percent of voters who supported someone other than Romney there were many motivated at least partly by anti-Mormonism. The point is that we can’t possibly know this based on information that does not even attempt to measure motivation.