It's an absolutely valid criticism of the Democratic party that they are splintered and seemingly without spine. Let's be honest, some of them are so used to being the minority party that they cower at the merest suggestion that they might be partisan (whereas their Republican counterparts proudly wrap themselves in it).
David Sirota suggests that they use Ben Nelson's techniques against him to start furthering a progressive agenda:
I spent my week on Capitol Hill, and specifically in the Senate, and I came away realizing the big problem Democrats face: Their caucus is held hostage by a small faction of people like Sen. Ben Nelson (D-NE) - conservative, Republican-appeasing senators who play hardball by always threatening to vote with Republicans to stop or water down mainstream Democratic legislation. For the Ben Nelsons, their behavior is very smart: in a narrowly divided Senate, threatening to undermine the Democratic Party pulls the Democratic Party towards their positions. This week provided a great example: with almost the entire Democratic caucus likely ready to support a resolution demanding the redeployment of American troops, the Nelsons - by letting Harry Reid know they will vote against even non-binding legislation to stop an escalation - have turned the debate into one about whether to support legislation by Republican Sen. John Warner, which avoids really using Congress's power to stop President Bush's surge. That's a huge shift and it displays real power.
[..]While on the Hill, I heard a number of Democratic staffers voicing anger at people like Sens. Russ Feingold (D-WI) and Chris Dodd (D-CT) for coming out and saying they will defy the Democratic leadership and vote against Warner's bill. Such complaints were also echoed to me by some of the outside anti-war groups. Their basic point was that those who vote against Warner are siding with the Republican leadership, who want to escalate the war. They, of course, have no similar criticism for the Ben Nelsons, who originally threatened to side with the Republican leadership had the Iraq resolution actually been strong (aka. not non-binding, or at least strongly worded), and whose pressure likely engineered Democratic efforts to defeat such strong legislation last week. It is this double standard that tells the real story of a critical imbalance.[..]
Progressives, of course, should cheer Feingold and Dodd for playing hardball, and not fall into the trap of trying to blame them for the potential failure of the Warner bill (which again, both does nothing because it is non-binding, and actually is a major legislative retreat from where the majority of senators already are). These two senators are forcing the Democratic leadership to think not only about the Ben Nelsons, but about progressives as well. They are making clear to everyone that the Ben Nelsons don't have some exclusive right to manipulate the Senate's narrow margins for conservative ends - and that this narrow margin, in fact, can be used for progressive goals. Suddenly, Harry Reid has to think about how he can keep Feingold, Dodd and other progressive senators from staying on board - and that means he might have to make concessions.