In addition to a Mormon problem, David Bernstein argues that Mitt Romney has an abortion problem as well:
[I]t really is true that Romney ran into incredible resistance to Mormonism, particularly among evangelical voters (and thus, particularly in the South). But again, I think that resistance really grew after. I don't think so many preachers would have been describing Mormonism as such an abomination at the time, if they had been more disposed toward Romney as a candidate.
From very early in the [2008] campaign -- or pre-campaign phase, really -- the McCain camp and their allies made sure that every religious conservative learned that Romney was lying about being pro-life. That's an absolute atrocity to those voters. It horrifies them.
Romney tried desperately to fight back, both insisting that he's always been pro-life, and also inventing a story about converting to the cause of the pre-born in 2004. It never took, which is unsurprising since he in fact was pro-choice (for these believers' definition, certainly) his entire political career, and thus his protestations were transparently bogus and unconvincing.
This sounds right; abortion, more than almost anything else, is the defining issue for social conservatives. If Romney were unambiguously pro-life, then evangelicals could tolerate his Mormonism. As it stands, Romney is clearly a "convert" to pro-life politics, which makes him virtually anathema to a significant portion of the Republican grassroots.
I should say that Romney's insincerity wouldn't really matter if he were elected president. Even if he didn't take active steps to restrict abortion rights, a President Romney would appoint Roe-hostile judges to the federal bench, nominate pro-life voices to the relevant federal agencies, and -- in all likelihood -- make rhetorical nods to anti-abortion activists. As Jonathan Bernstein is fond of saying, we vote for political parties -- not just candidates -- and the GOP has a fairly strong anti-abortion consensus.