The problem is that the next election is 2008, and its not as though the Democrats have a huge hold over the country right now.
When Bush leaves office (2008) is when the Democrats would have a chance to solidify their hold on Congress enough to get away with it.
Therefore, for political reasons, they aint gonna do it.
In your post, you imply that Congress is outside of control of the party and that they can do their own thing. Thats not a correct view - they better listen to what Howie "Yeahhhhh" Dean wants if they want to be supported by the party.