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I don't expect Lieberman to be a Democrat. However, that's only one issue, and it's not the biggest one -- there will be people defecting on both sides, in most cases.
The big issue is who will get to decide the chairmanships and majorities of the committees. If Lieberman sticks to his word, that means it will be the Democratic Caucus. Which means the Democrats will be able to control the agenda -- and also begin hearings, subpoaena documents, etc.
It will be very interesting to see how Lieberman acts for the next six years.