Yesterday, Senate Democrats made the decision I predicted back in May and June and let Joe Lieberman keep his committee chairmanship.

The reason is simple. As the New York Times notes, “Democrats want to avoid driving Mr. Lieberman into the Republican fold.”

Lieberman’s voting record currently puts him in the middle of the Democratic caucus. If he left the party, he would likely move substantially to the right. The political scientist Timothy Nokken has found that “Democrats who become Republicans… start to vote like Republicans” and that House members’ changes in voting behavior are especially pronounced on amendment and procedural votes. Given that the Democrats will repeatedly need Lieberman’s support on cloture votes to end GOP filibusters, their decision seems to be a wise one.