In a recent post, Jonathan Chait rightly calls our attention to the Pew survey released July 16 that showed how voters rate political parties’ ideologies. While I agree with Chait’s interpretation of the data he cites, I want to underscore the significance of some other information in the survey—namely, where voters identify themselves in relation to the parties.

On the whole, 58 percent of voters see Democrats as liberal or very liberal, while 56 percent see Republicans as conservative or very conservative; no surprise there. But voters now place themselves much closer to the Republican Party than to the Democratic Party on this left-right continuum. Indeed, the ideological gap between the Democratic Party and the mean voter is about three times as large as the...