If we do get a 49-49-2 Senate

[ag]

They'd have to negotiate. This is not a statutory issue - it would be entirely the matter of coalition politics.

[ag]

Lieberman can't dump Dems that easily. At least not immediately. Had he not pledged in no uncertain terms that he'd caucus w/ Dems, he wouldn't have been elected, no matter what (Sen. Chaffee could tell you all you need to know about it).  Going back on that pledge immedeately, giving the Senate to the Reps straight after such an election would only make sense for a CT senator if he has already decided he is never again running for office in his state. Reps would have to give him an iron-clad promise of, at least, the Vice-Presidencial candidacy in 2008, which, obviously, they can't. Even then, he'd be an idiot to do this: since Senate Dems would remain out of power under this scenario (something that most citizens of his state would be very unhappy about), everything that goes wrong in this case would be politically the liability of one and only one man: Sen. Lieberman. He'd be poisonous by 2008, he wouldn't be able to walk down a street in Hartford without being spit on.  CT is strongly moving into the Dem column - there is no reason for any CT politician to become A Rep. The probability of him voting for Dem committee chairmen in January is very close to 1 (well, unless I am mistaken about him not being an idiot).

That said, if Dems turn out to be a major disappointment later, Lieberman would always have a gracious way of deserting them: he wasn't elected as one, he indeed owes them nothing. I am far from certain Lieberman stays a Democrat in 2008 - but he doesn't have much of a choice now.