For him, the most salient demographic change from 2008 to 2012 was the drop in white voters, and specifically, downscale, Northern, rural whites. Its unlikely that these voters were liberal, and if they were in the electorate, theres a good chance they would have broken for Romney in large numbers.
Assuming that everybody who's "unlikely to be liberal" is conservative in the sense many of us use it, is a mistake. It's the sort of mistake people make with Perot voters. They certainly weren't liberal in the Berkeley-Cambridge-Manhattan-San Francisco sense and would be open to vote for a Republican, but it certainly doesn't follow that they're looking for the most conservative of Republican candidates. There are things about conservatives and Republicans that they also may not like. So, yes, Romney wasn't the right candidate to win them over, but the right candidate wasn't necessarily the most conservative one, and probably wasn't in the race either.
Bump.