Washington (CNN) -- Senate Republicans again Tuesday blocked Democratic legislation that would require greater transparency into who is behind much of the secretive, often negative campaign advertising filling the airwaves this election season.
Supporters of the DISCLOSE Act mustered 53 votes in an effort to advance the bill, but fell short of the 60 needed to break a GOP filibuster. Democrats unanimously supported the bill, while Republicans voted unanimously against it. The legislation, which needs 60 votes to succeed, failed on a 51-44 vote Monday.
The legislation would require certain tax-exempt groups involved in political advertising -- which currently are not required to disclose the names of their donors -- to release the names of those who give donations of $10,000 or more. A similar vote Monday ended with the same result.
"During this election, outside spending by GOP shell groups is expected to top $1 billion," said Senate Majority Leader Harry Reid, D-Nevada, in a floor speech. "The names of these front groups contain words like 'freedom' and 'prosperity.' But make no mistake: there is nothing free about an election purchased by a handful of billionaires for their own self-interest."
Democrats say the requirement is a much-needed response to the U.S. Supreme Court's Citizen United decision, which opened the doors to large amounts of campaign spending by corporations, labor unions and other groups.
The political advertising all those donations buy "threatens to drown out the voice of middle-class families in our democracy," argued Sen. Sheldon Whitehouse, D-Rhode Island, a chief sponsor of the bill.
Senate Minority Leader Mitch McConnell, R-Kentucky, led the GOP charge against the bill and accused Democrats of crafting it to favor labor unions -- traditionally Democratic allies -- over other types of donors.
"This legislation is an unprecedented requirement for groups to publicly disclose their donors, stripping a protection recognized and solidified by the courts," said McConnell, who has advocated for more disclosure in the past so long as it applies to all big players in campaign financing, such as labor unions and trial lawyers.
McConnell cited a recent Wall Street Journal analysis that found labor unions spent $4.4 billion on campaigns between 2005 and 2011.
"Predictably, unions are exempted from the kind of disclosure Democrats now want to impose on others," McConnell said.
Democrats disagreed with the Republicans' assertion that labor unions are exempt from their legislation. However, the concern about labor unions appeared to sway key Republicans such as Sen. John McCain of Arizona, a longtime vocal advocate of campaign finance reform, who said he would vote against the bill for that reason.
"By conveniently setting high threshold for reporting requirements, the DISCLOSE Act forces some entities to inform the public about the origins of their financial support, while allowing others -- most notably those affiliated with organized labor -- to fly beneath the Federal Election Commission's regulatory radar," McCain said.
McCain argued that local union chapters would not have to disclose payments from individual members if the amount was less than $10,000.