The most insulting thing to conservatives about this argument is that it says that conservatism and electability are mutually exclusive. History says otherwise. Ronald Reagan, the standard bearer of modern conservatism, was unelectable. He was too conservative, a reactionary, and a deluded old fool who would bring about a nuclear war.
Of course he won two back-to-back landslides (44 and 49 states, respectively), and he did it, not by being a moderate, or seizing the center. No, on the contrary, Reagan did what Mark Steyn best describes. As Steyn points out, Reagan did not win by moving to the center. Rather, he moved the center towards him. That, Steyn says, is why Reagan was great. He knew how to speak to independents and centrists without losing his principles in the mix. He was a conservative who could move the center.
Romney has no such record. Candidates who have had success doing this are the unelectable Rick Santorum (in Pennsylvania, every bit as blue as Massachusetts, where he was successful in doing it four times) and the unelectable Michele Bachmann (in Minnesota, also every bit as blue as Massachusetts, where she was successful in doing it three times).