Bernie Sanders is blisteringly angry at the Obama Administration over its massive proposed trade deal, the Trans Pacific Partnership.

In a letter to the US Trade Representative Michael Froman this week, the progressive Vermont senator and potential 2016 presidential candidate blasted the administration for, he says, not providing enough access to the agreement in its current form, the Huffington Post reports.

critics like sanders are complaining about the process, but that's because they're worried about the outcome

Noting that the deal is mammoth, comprising countries representing 40 percent of the world's GDP, Sanders writes, "I have been very concerned that up to this date the text of this agreement has not been made public. The only text that I am aware of that has been made public so far has been through leaked documents, and I find what I read to be very troubling."

Of course, members of Congress do have access to the agreement, and they also have had access to US trade officials.

"USTR has held nearly 1,600 briefings and consultations with Members of Congress and their staffs," USTR Spokesman Matt McAlvanah said in a statement, adding that Congress members will be able to "instruct the Administration on the objectives to seek in trade agreements, the procedures for consultation, and the process that Congress uses to consider trade agreements."

But not all congressional staff members have access, and those staff members, as Huffington Post reported, can't hold on to their own copies of the agreement and study and analyze it. Sanders requested that this restriction be lifted so staffers and outside experts can assess the agreement.

But Sanders isn't just angry about that; he's angry about corporate leaders' place in the negotiations.

"Please...explain why you think it is appropriate that the representatives of the largest financial institutions, pharmaceutical companies, oil companies, media conglomerates, and other major corporate interests not only have access to some of these documents, but are also playing a major role in developing many of the key provisions in it," he writes.

Sanders is referring to the trade advisory committees, a system Congress set up in the Trade Act of 1974. Members of the advisory committees, like Congress members, "are shown all US proposals before they are presented to our trading partners," says a USTR official, and "offer important guidance" in the process.

There are around 700 members on those councils providing input on TPP, according to the USTR website, and the members are listed on the USDA, USTR, and International Trade Administration websites. Those members all have equal access to the proposals, a USTR spokesperson tells Vox. And they're not all from the business world. However, many are. While those committees include members from academia and labor unions, a 2014 analysis from the Washington Post showed that 85 percent were from businesses or trade groups.

This matters right now because Congress is poised to take up legislation on Trade Promotion Authority, also known as "fast-track" authority. Congress periodically renews TPA to allow administrations to negotiate trade deals. Under TPA, Congress sets out some guidelines on how the US should negotiate a trade agreement. Then, when an agreement has been reached, the deal cannot be amended — it goes to a straight up-or-down vote. One big point of this system is to allow the US to negotiate without the threat of Congress changing an agreement after it has been, at long last, reached with trading partners.

There is a strong case for not making the US's full positions public.

"As soon as you reveal your position and put it in print, then it's much more difficult to modify it and be flexible later," Gary Hufbauer, senior fellow at the Peterson Institute for International Economics, explained to me in November. "It's a negotiation, and everyone has to compromise to some extent."

But critics like Sanders aren't really just complaining about the process. What's driving them are worries about the deal's outcomes. There are, first of all, fears that further opening up trade could threaten some workers' jobs. But the outcomes could be even more far-reaching.

What's being negotiated here isn't just lifting a few tariffs. TPP covers a wide swath of issues both directly and indirectly linked to trade — the environment, labor laws, and intellectual property, to name a few. Secrecy may make negotiating easier, but what's being negotiated is simply huge. That's what makes some members of Congress so nervous about it, and that's what to expect in the coming months of fighting about trade deals on Capitol Hill.

Updated. This article was updated to include comment from USTR Spokesman Matt McAlvanah.