Didn't you learn from school that the religious wars of medieval Europe weren't exactly the greatest thing in the world?
Well, the fact that the Religous Right has gotten as far as it has in American politics might indicate that, no, in fact, they did NOT learn that from school.
Which is not to say that there aren't Americans who are appalled by the fanning of the flames of religious bigotry, but I think most of them, if not the overwhelming majority, were probably already voting Not Republican to begin with. So you need to somehow come up with a situation where people who were comfortable voting for Sarah Palin(whose pastor had actually been involved in LITERAL witch-hunts) in 2008 suddenly decide that villifying other faiths is a step too far.
Maybe if you can somehow get good ol' fashioned anti-Catholicsm to rear its head on the religous right, and that gets linked to Islamophobia. Say some GOP-friendly preacher drops his guard and says in the same breath that Catholics and Muslims will burn in hell, and that obviously pisses off Catholics, but they can't really say that he's wrong to attack Catholics without also saying he's wrong to attack Muslims, since it's impossible to ignore the latter comment.
Or, given that it's Romney we're talking about, maybe throw anti-Mormonism into the witch's brew. Assuming that wikipedia is accurate...
Muhammad and Joseph Smith
The Church of Jesus Christ of Latter-day Saints considers Muhammad to have received a portion of God's light, and that moral truths were given to him to enlighten nations and to bring a higher level of understanding to individuals.[59] However, it does not consider him to have been a prophet in the same sense as modern-day LDS prophets nor ancient prophets found in the Bible and Book of Mormon, and does not accept the Qur'an as scripture. Conversely, Islam does not accept Joseph Smith as a prophet, for it believes Muhammad to have been the final prophet of God to humankind.[60] It equally does not accept the Book of Mormon, or any of the other Latter-day Saint Standard Works, as the Qur'an is believed to be God's final revelation for all time, and for all people.
This might work best with a third-party Christian conservative candidate, untethered to GOP electoral considerations, who brings up this aspect of Mormon doctrine, juxtaposes it with a bunch of texts from the Koran seemingly calling for the murder of non-Muslims, photos of 9-11 etc, and then demands that Romney respond. Things go from there.