"after exhausting all means..." Hah-hah, that's a good one. Sorry, I generally try not to be obnoxious in my politics, but sometimes I lose to the dark side.
Lieberman was vulnerable because a lot of his values aren't terribly different from those held by conservative senators. He supports No Child Left Behind. He used to be a strong environmentalist, but he voted for the big energy bill that benefitted the oil companies in 2005. He sits the fence on big issues like tax cuts and gay rights. He'll make a key vote here or there and then do a lot of smaller ambiguous things that make you wonder what he really believes. The Iraq War might have been the rallying point, but I think that what differentiates him from Feinsteinn is that you know the difference between Feinsteinn and a Republican. You don't always know with Joe.
I still support my senator, Maria Cantwell, even though she voted for the war...but I would have voted Lieberman out, too.