Brad DeLong points to a piece by Michael Tomasky in The Daily Beast.
As a rule, Republicans are better at this [reacting to emotion rather than reason] than Democrats. The latter often still base their appeals on reason. Not because they’re superior human beings, but because there is something about the liberal brain that wants to believe that if there is a problem in the country, the experts will study it and offer a solution and the politicians will implement it. This brand of “liberalism,” which prizes empiricism and pragmatic problem solving, was for many years the default position in American society. The conservative movement, in contrast, rose up in opposition to this alleged dictatorship of the empirical. So conservative appeals by their very nature are more tightly constructed around denunciations of the liberal status quo—which is to say, around emotion.
This means, to put it more simply, hating on liberals: feminazis, socialists, freedom haters, French apologists, and so on. A big part of the definition of a true and fully engaged conservative today is that that person really, really hates liberals.
But Romney just doesn’t hate liberals. You can tell he just doesn’t. Then he tries to act like he does, and you can tell it even more. He didn’t grow up in that kind of atmosphere—his dad, though a Republican, was plenty liberal by today’s standards—and later in life he obviously didn’t govern in that kind of atmosphere. And so liberal hatred is simply not woven into his DNA.