In a blog post about an unrelated topic (Eliot Spitzer), John Podhoretz explains a point that, while certainly not original, is rarely stated so clearly:
Too often, people find it difficult to separate their own ideas about politics from their moral expectations. Democrats and liberals slip far too easily into a conviction that the Republican and conservative focus on equality of opportunity and the benefits of the market is merely a cover for greed and power dominance. Republicans and conservatives, likewise, believe the Democratic and liberal elevation of the government’s role in solving social problems is merely a cover for a bottomless hunger to arrogate and centralize political power. They are not content to believe their opponents are wrong. Rather, they are sure their opponents think exactly the same way they do and, therefore, that they are acting from malign intent rather than from a different perspective on how the world works best.
Conservatives tend to view the world through a moral framework, and this makes them susceptible to believing that others are immoral because they do not do so. Liberals tend to view the world through a framework of compassion, and this makes them susceptible to believing that others are heartless because they do not do so.