Making the claim that Romney's religion is a good reason to vote against him can be unpopular in liberal circles. We ... (I claim to be one of the "liberals".) ... tend to be tolerant of people's individual choices and we feel that differences should not be held against people.
Romney's case is different. Hear me out.
First, Romney is running for President. If he was elected, his values would change the direction of the country; even the world. His values are a genuine, legitimate concern for all of us.
Second, I accept that Romney is deeply committed to the Mormon Church. I live among them. I see the signs that tell me that Romney is just like the Mormon bishops, stake presidents, and general authorities who run the state of Utah with their unflinching certainty that they are right and nobody else is. (See my article, Where Romney Learned His Lying Ways)
But here's the clincher: Mormons really do think it's OK to lie to us. The Lord told them it's OK. They have a long, rich history of lying to anyone outside the Mormon Church and Romney, as a believing, committed Mormon, is an heir to this history.
Take polygamy, for example. The Mormons themselves will now admit that Joseph Smith made polygamy part of the Church doctrine thirty years before they admitted it due to the overwhelming evidence of what everyone could see. But during that thirty years, they denied it (To begin, even to the lower ranks of their own people.) for the very practical reason that it was illegal and admitting it would have created problems for them. In a very real way, Joseph Smith died to protect the secret because the direct reason he was captured and killed by his enemies was that he ordered the destruction of a newspaper created to publish those secrets. (Mormon doctrine STILL commands Mormons to practice polygamy today. The 1890 Manifesto only commands Mormons to obey current laws. The doctrine has not changed, but Mormons actively work to create a false image that it has.)
Take the Mountain Meadow Massacre. For a hundred and fifty years, the largest single massacre of pioneers in the settlement of the West was hidden by Mormons and covered up by a massive lie. One courageous Mormon, Juanita Brooks, finally brought it out into the open. (Not all Mormons are liars, even though it's part of the culture.) They're still lying about it. When Will Bagley published his definitive study (See my review, Lying for the Lord.), the Mormons found Mormon historians to publish a competing apology so they could salvage as much as they could.
Romney actually does deeply believe that it's OK to lie to us. Understanding this explains a lot.