Bush is about to get his number one priority through Congress, a move that could be stopped by Edwards, Obama, or Clinton, especially the latter two. This is the move to implement retroactive immunity for telecom companies who spy on Americans and violate core constitutional principles.
All that is required to fight this is for Clinton or Obama to put the glare of the Presidential spotlight in the Senate. To, you know, lead. All three campaigns are well-aware of this fight, and at least Clinton and Obama have been completely unresponsive.
South Carolina is probably locked up for Obama, and since the fight now is over swing liberals and the campaign is about to move national, this would be a smart political move. You get more national attention by confronting Bush in DC in a dramatic filibuster than you do with bland paid media. PT Barnum said if you want to attract a crowd, pick a fight, and Democrats who vote in Democratic primaries like people who fight Bush. Hell, everyone likes people who fight Bush.
Meanwhile, I see limited or marginal reasons to care who wins the primary. I respect supporters of every candidate. Still, if you want to persuade me to drink your kool-aid, get your candidate to help Dodd protect the damn constitution already. And then I'll drink a big cup of whatever it is you're offering, and so will lots of us.