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I think a big part of the Democrats motivation to be bipartisan is that they naively think that, since they gave Bush pretty much everything he wanted, the Republicans will follow their lead and do the same for Obama. They clearly learned nothing from the Clinton years. The Republicans pay lip service to bipartisanship but I can assure you they are not going to roll over and play dead like the Democrats did under Bush.
Also, the Democrats fail to understand that one of their biggest weaknesses is the perception that they cave in too quickly and are therefore not to be trusted to defend the national interest. Given how they fail to even defend their own party against a turncoat like Lieberman, I can understand why this perception lingers. Ditching Lieberman would have sent a message to the electorate that the Democrats are no longer appeasers and it would have been a big positive for them.