"In 2001, Democrats were insane with fury that Lieberman wasn't the vice president."
Well, no. First, "insane with fury" and "livid with outrage" (the latter being the accurate description if one talks to Democrats instead of listening to Rush) are not the same thing. Second, the Democrats were angry that the election was decided not by a thorough recount, but by the Supreme Court. Lieberman, personally, was not part of the equation.
His positions haven't changed significantly since then, so what has?
He has become a big Bush supporter. Bush has been shown, by his own performance, to be an incompetent executive and a political opportunist. He who lies down with dogs gets up with fleas.