Actually, it's worse. Sure, they might see there is a risk of losing independent votes by running a filibuster.
But every democrat will lose in november if their base actually rejects them. They have to take that into account.
So the louder the base is, and the more likely it seems that their base will hold them accountable, the more likely it is that individual senators would vote for a filibuster in the hopes that just doing so would turn the tide (HEADLINE: "Democrat Filibuster raises new questions about Alito's Qualifications"), and that in any case the independents would forget it by november, and still vote democrat because of all the other stuff.
Because they KNOW that the base won't forget -- the only question on that side is will the base concede elections to republicans to punish their own wayward democrats.
Now, the recent polls for Senator Lieberman are encouraging on that front, as the left's rejection of him doesn't seem to be reflected in his poll numbers. That might strengthen the resolve of some democrat senators to do what is right, rather than what the far left order them to do.
Right, so this is why foot dragging and stonewalling is so important to them. Anything that they can do to delay this enough so that it won't come up until after the elections, with their hopes that the Senate would either turnover or at least the majority would lessen.
Or the Democrat Senators might think "Where else is the Left going to go but to us"? The Left is far too politically rabid to stay home on election day. There whole sense of self depends on "getting" the Conservatives. No matter what the Dems do, the base will be there for them. They hate us too much to stay home on election day