Democrats are merely playing politics. They know that the bill isn't going anywhere, and Senate Majority Leader Harry Reid himself voted against the measure. The Washington Post explains this, saying that Reid's move is "a procedural tactic that will allow him to call the measure back up for another vote on Tuesday." The Democrats don't particularly care about this issue - they only care that it gets covered in the press and the political warfare that they're gleefully waging on "corporate money."
Senate Minority Leader Mitch McConnell has been on a tear against the DISCLOSE Act and in favor of political speech lately. His op-ed in the USA Today recently put it succinctly:
Billed as "reform," the measure is an attempt to identify and punish political enemies, or at the very least, intimidate others from participating in the process — an effort that's already underway.
The Obama administration has tried to single out its critics through federal agencies such as the IRS and the Federal Communications Commission, and even through a proposed executive order aimed at denying government contracts to opponents. The president has used selective disclosure not as a tool of good government, in other words, but as a political weapon.
Democrats are going to hammer home these themes throughout campaign season. They're long done governing - they're there to settle political scores ahead of the 2012 election.