Put yourself in their shoes. Imagine being a religious liberal. You view the Bible primarily as a fable. You check the "Christian" box when asked your religion, but you don't associate with anyone who takes the Bible literally. You hope your followers vote against the Republican candidate. And then here comes Mitt Romney as the one chosen to oppose the "chosen one."
This presents a real dilemma for those on the religious left. It is not part of their practice to articulate biblical doctrine in too much detail. It all must be kept incredibly vague and inclusive. The minute a person on the religious left allows himself to get too specific about doctrine, he starts to feel way too much like an absolutist. Somebody gets left out.
Many on the religious left today are "Christian" in the same sense that America is a "Christian" nation. It is a label that gains you a following and a measure of respectability and status. It is not intended to mean that you believe in Christianity literally, say the way that Muslims or Mormons believe their respective religions present literal history and literal truth. For the religious left, it is more of a figurative approach to their "faith." Christian "mythology" is meant to teach some good life lessons, but not to prescribe the rightness and wrongness of anyone else's doctrine.
Mitt Romney has committed his soul to a religious organization that has a specific history and many controversial doctrines. This poses a problem for the religious left. How do they discredit the man's policies without being asked questions about the history or doctrines of his religion? If they are asked specifically about those things, how in the world are they to respond? Those issues are way outside of their comfort zone.
The biggest area of nervousness, however, really has to do with their own "faith." If they allow themselves to compare the history of Mormon doctrine to the history of Christianity, they are allowing themselves to view Christianity as more than a fable. Everyone knows that Mormonism is not a fable. It is a fact of recent history, just as the Jehovah's Witnesses are a fact of recent history.
Those on the religious left don't want to wrestle with this essential aspect of Christianity. It makes it all too real. It brings heaven and hell into the picture, as well as sin and the Savior. It leads people to say there are true prophets and false prophets. Do you honestly think, then, anyone on the religious left would ever identify Mormon founder Joseph Smith as a false prophet?
You won't hear the religious left label people as false prophets because that is too exclusive. That results in "right and wrong." In their mind, that is not an option. It simply is not politically correct or spiritually correct to be that narrow in your thinking. The only exception is when it comes to those who interpret Scripture literally and take Jesus at His Word about being the only Way to heaven. For some illogical reason, the religious left view these Christian "literalists" as fair game for criticism. Hmm. So much for a principled position.
In the end, what is one to do with Mitt Romney? If you are part of the religious left, you tiptoe very gingerly around this candidate. If he gets the nomination, you freely criticize his financial policies and other policy matters. But you steer clear of his Mormon beliefs. That topic will only make a religious liberal start sweating.