Today the Democrats in the U.S. Senate voluntarily allowed the DISCLOSE Act to fail, again. This is the bill that would require disclosure of the sources of some independent expenditures in races for U.S. Congress and President.

The Democrats allowed the Republicans to filibuster the bill. The vote to end the filibuster (called cloture) was 51-45 in favor of ending the filibuster and thus allowing a vote on the bill itself. The chair then declared that the cloture motion failed, because it requires a 60% affirmative vote.

At that point, the Democrats could have invoked the "Constitutional Option" and have challenged the ruling of the chair.  That ruling could be overturned with a simple majority vote, as it was last year when Harry Reid invoked that option (also known as the "nuclear option") to disallow introduction of a series of amendments to a bill about Chinese currency manipulation.  The Hill reported on Oct 6, 2011:

In a shocking development, Senate Majority Leader Harry Reid triggered a rarely used procedural option informally called the “nuclear option” to change the Senate rules.

Reid and 50 Democrats voted to change Senate rules unilaterally to prevent Republicans from forcing votes on uncomfortable amendments.

Reid’s coup passed by a vote of 51-48, leaving Senate Republican leader Mitch McConnell (R-Ky.) fuming.  The surprise move stunned Republicans, who did not expect Reid to bring heavy artillery to what had been a humdrum knife fight over amendments to China currency legislation.

The Democrats use "heavy artillery" on bills of marginal importance but wield a  rubber sword when it comes to campaign finance reform.