Every political community includes some members who insist that their side has all the answers and that their adversaries are idiots. But American liberals, to a degree far surpassing conservatives, appear committed to the proposition that their views are correct, self-evident, and based on fact and reason, while conservative positions are not just wrong but illegitimate, ideological and unworthy of serious consideration. Indeed, all the appeals to bipartisanship notwithstanding, President Obama and other leading liberal voices have joined in a chorus of intellectual condescension.

It's an odd time for liberals to feel smug. But even with Democratic fortunes on the wane, leading liberals insist that they have almost nothing to learn from conservatives. Many Democrats describe...