James Kirchick writes in the Daily News:

hysteria over ideological apostasy is something that has long followed Lieberman, who has differed with the more doctrinaire bloc of the Democratic Party on a handful of key issues. But it’s especially rich that the left wing is accusing Lieberman, of all people, of “betraying” the party or its principles. For if anyone has done betraying, it’s not Lieberman, but the ones launching that very accusation.