Liberal commentators like to spotlight rifts in the Republican party and self-destructive fights among various flavors of conservatives. In this they often have legitimate raw material to work with.

But I haven’t seen so much introspection in those quarters when liberals do similar things. Case in point: Arkansas, where New York Mayor Michael Bloomberg’s Mayors Against Illegal Guns is running ads against Democratic Sen. Mark Pryor, who voted against the Manchin-Toomey gun control proposal. These ads could hurt Pryor and, as Time’s Michael Scherer points out, reduce Democrats’ chances of holding onto a Senate majority in the 2014 election.