A coalition including civil liberties groups and government whistleblowers has come out against a Senate bill responding to the government surveillance and data collection revealed by former National Security Agency contractor Edward Snowden.

Many observers see the current Senate version of the USA Freedom Act as the most likely to succeed. But a letter released by the group Monday argues that the language in the bill is too murky and could actually codify some controversial government programs while failing to provide meaningful prohibition against mass surveillance.

"The USA Freedom Act has significant potential to degrade, rather than improve, the surveillance status quo," the letter warns. "At best, even if faithfully implemented, the current bill will erect limited barriers to Section 215, only one of the various legal justifications for surveillance, create additional loopholes, and provide a statutory framework for some of the most problematic surveillance policies, all while reauthorizing the Patriot Act."

Signers of the letter include NSA whistleblowers William Binney and Thomas Drake, as well as Pentagon Papers source Daniel Ellsberg and groups including Progressive Change Campaign Committee, the Sunlight Foundation, Restore The Fourth, and Fight for the Future.

But notably absent from the list are some of the big-name civil liberties groups -- including the American Civil Liberties Union, the Electronic Frontier Foundation, the Center for Democracy & Technology and New America Foundation's Open Technology Institute -- who have signed on to a letter endorsing the version of the bill introduced by Senate Judiciary Committee Chairman Patrick Leahy (D-Vt.). Major tech companies have also endorsed the bill through trade groups and industry coalitions.

The legislation includes limits on surveillance under Section 215 of the USA Patriot Act -- the part invoked to justify the bulk collection of domestic phone meta-data -- as well as additional transparency provisions and the creation of a special advocate for civil liberties within the secretive court that overseas surveillance decisions. But the bill does not address surveillance or data collection occurring under other authorities, including Section 702 of the USA Patriot Act and Executive Order 12333.

Neema Singh Guliani, legislative counsel with the ACLU, says the organization "conducted a careful analysis of the bill" and believes on the whole it is a step in the right direction, although "not perfect."

Kevin Bankston, the policy director at the New America Foundation's Open Technology Institute, says he sympathizes with the group opposing this version but believes this bill is the best path forward. "I agree with the signers of today's letter that USA Freedom Act doesn't go nearly far enough in addressing all of the worst NSA surveillance practices," says Bankston. "But I also believe this bill is a critically important first step in the reform process that would end the NSA's bulk telephone records program while giving us much more transparency and accountability when it comes to government surveillance overall. "

Those opposing the Leahy version of the bill argue it may not actually end bulk surveillance programs. "Given the several broad legal authorities claimed as justifications for mass surveillance of United States persons and non-United States persons," the letter reads, "it remains unclear if the Senate’s USA Freedom Act would end any of the Intelligence Community’s clandestine programs to surveil Americans."

Sascha Meinrath, directer of X-lab at the New American Foundation and the founder of the Open Technology Institute, is skeptical that the bill would effectively stymie bulk collection and signed on to the letter opposing the bill as an individual. Even experts on the matter, he says, have trouble determining the actual policy outcomes of the legislation because of the measure's "nebulous" language.

"People have different interpretations because the bill was written to create different interpretations," Meinrath says, adding that what opponents of the bills perceive as vagueness could be abused.

Julian Sanchez, a senior research fellow working on surveillance issues at the Cato Institute, says many supporters of the bill would likely agree with the criticisms in the letter but disagree about tactics for addressing the broader issue of NSA surveillance.

"The current version of the bill focuses on Section 215 and similar authorities used to obtain communications metadata, without addressing many other authorities that privacy advocates want to reform," he explains. "So a great deal of the difference here is between groups who want to accept this partial fix where there’s consensus and then move on to addressing things like surveillance of international communications under Section 702 in another bill, and groups that worry this is going to be it — Congress is going to pass one 'surveillance reform' bill and call it a day."

This isn't the first time the USA Freedom Act has drawn strong debate. An earlier version of the bill on the House side was repudiated by many civil liberties groups and tech companies after the bill that made it to the floor was substantially altered from the version passed out of committees. The House version passed, but with half of its earlier sponsors voting against it on the floor.

Correction: This story has been updated to correct Daniel Ellsberg's role in revealing the Pentagon Papers. He was a source, not a journalist.