Anderson explains this dynamic in the comments section:
[T]he opinion surveys in which large percentages of Evangelicals rejected Romney on account of his religion had no problem with a Jewish practitioner - meaning here, not simply a Jew ethnically or culturally, but as a matter of religious practice and affiliation. That was fine. Mormonism was regarded as specifically offensive because it was either pagan, polytheistic, or heretical in the specific sense of spreading false doctrine in the name of the faith. It was not the case that they required a person of their religious beliefs. Mormonism was specifically out of bounds as a faith that actively led people astray because it claimed to be Christian but was actually something deceptive. Not just false, but deceptive.
And I noted in a subsequent comment how an analogous dynamic existed during the American Founding:
Your comment about "deception" is important. I often hear evangelicals interchange "Christian" with "Judeo-Christian" in terms of "foundations" in which they support. Many of these are the same folks whose support for Israel and the "Jewish people" has something to do with end times prophecy. Note, I'm not an "anti-zionist" (I tend to support Israel as well); I'm just making an observation. When I press them for "definitions," "Judeo-Christianity" usually means orthodox Christianity where Jews get to tag along for fun or for some *other* special reason.
I think this again relates to the Founding. From John Adams to Thomas Jefferson, they more likely defended their understanding of the "true religion" in which they supported under the auspices of "Christianity" and not some anti-Christian Deism (ala Thomas Paine). But their understanding of "Christianity" was unitarian, and tended to be naturalistic, rationalistic, and generic in its moralization of the Christian faith (i.e., if you were a good person and acted like Jesus -- the world's greatest moral teacher -- you were a "Christian" regardless of your views on original sin, Trinity, Atonement, etc.).
It wasn't exactly Mormonism; but the same "deception" issue was involved. By the time unitarians Richard Price and Joseph Priestley (whose influence on the "key Founders" cannot be emphasized enough) began to speak out, the "orthodox" critics responded with the same "this isn't Christianity, it's a false system that calls itself 'Christianity'" to them as they today do with the Mormons.
For that and a number of other reasons I think Mormons (and other "outsider" religious groups) should feel an affinity for the American Founding.