Leiberman is an independent. He caucuses with the Dems. If, he were to caucus with the Republicans, that is where the damage will be done, shifting the caucus to a republican majority. I do not believe that he would do that, he is a democrat by nature, but he will have considerable power as a needed swing vote for both sides ICO a 50/50 tie. Remember, the VP as president of the Senate has the tie breaking vote.