More importantly, Citizens United opened the floodgates for super political action committees (PACs). Super PACs cannot contribute directly to a candidate but can run advertisements in favor or against a political candidate. Between 2008 and 2012, outside spending increased by more than 225 percent and totaled approximately $1 billion in the 2012 elections. Only a record-low 41 percent, out of a total $41 billion spent on television and radio advertisements by independent groups, was fully disclosed, according to the Center for Responsive Politics.

The American Civil Rights Union (ACLU) praised the Murkowski-Wyden bill, saying that it does many things better than competing proposals, but warned of the unintended consequences of donor disclosure.

“There is no question the public has a legitimate interest in knowing who is corruptly spending scads of cash trying to influence voters and lawmakers. Absent anonymity, some donors—on both the left and the right—will simply not donate out of the legitimate fear they will be harassed or retaliated against for their advocacy,” wrote Gabe Rottman, legislative counsel at the ACLU.

Conservatives groups also came out against the bill.

“When liberals talk about ‘transparency,’ that isn’t what they mean,” Cleta Mitchell, chairwoman of the American Conservative Union Foundation, said in a statement. “What they really want and what this bill provides is a target list of conservatives who have the temerity to contribute their after-tax dollars to support candidates and issues the left hates.”

The Center for Competitive Politics criticized the bill, saying that the lawmakers introduced the measure without any consideration of whether “it is overly burdensome and/or deters political speech protected by the First Amendment.”

“The bill would radically expand the reach of government regulation on speech critical of elected officials and force many, if not nearly all, advocacy groups to register and file burdensome reports with the federal government,” the organization said in a report.

Free-speech advocates have been the major obstacle for disclosure reform on grounds that anonymous political speech should not be infringed at any level. Sens. Mitch McConnell (R-Ky.) and Ted Cruz (R-Texas) have constantly opposed any type of campaign finance reform.

McConnell met with House Republicans last December warning them not to sign on to any bipartisan initiative requiring super PACs to disclose information about their donors more frequently. Cruz said last year that current campaign finance laws should end to allow unlimited donations to campaigns.

“Whenever Congress acts in the area of political speech, there’s a touch stone for everything we do, should be the first amendment to the Constitution. And I think that the public should be particularly skeptical when you have elected politicians of either part enacting rules limiting the ability of public citizens to criticize the behavior of their elected officials,” Cruz said recently during a Senate hearing on campaign finance reform.