Washington (CNN) Two copies of the biggest free trade deal in history are sitting in reading rooms -- one at each end of the Capitol.

The document is classified. Only members of Congress and staffers with security clearance can access it. And they can't make copies or even carry their own handwritten notes out the door.

This is how trade negotiations work. Fearful that they'll undercut their own negotiators, leaders of the countries involved don't want the details of what they're hashing out revealed until the full package is completed. And it's at the heart of the biggest criticism opponents of the deal have made publicly: the secrecy surrounding it.

"I've worked with the Clinton administration and I've worked with the Bush administration. And this administration is more secretive," said Thea Lee, the deputy chief of staff for the AFL-CIO, which is spearheading the left's opposition to the deal -- even though it's being negotiated by a Democratic administration under President Barack Obama.

The pressure for more details to be made public isn't just coming from the deal's opponents. WikiLeaks recently announced a $100,000 bounty for the Trans-Pacific Partnership's negotiating text -- an offer that comes after the organization has published several of the deal's chapters and proposals over the last two years.

A massive deal

The Trans-Pacific Partnership is a 12-nation deal that would link 40% of the world's economy -- including the United States, Japan, Australia, Canada and Mexico.

The United States has been involved in the negotiations since early in Obama's first term. But all that work could be undone if a key vote to put the deal on the so-called "fast track" doesn't go the administration's way in the House on Friday.

Lawmakers are set to vote on Trade Promotion Authority. It's a measure that greases the legislative skids for the Trans-Pacific Partnership by guaranteeing it an up-or-down vote with limited debate and no amendments -- as long as a list of Congress's negotiating objectives are met.

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That bill looks set for a close vote, with most Democrats opposed to advancing another trade deal that they fear could cost jobs and drive down wages, and some conservatives hesitant to hand Obama any new authority and angry that the legislation also includes aid for displaced workers. Thursday afternoon, the House voted 217-212 to approve a rule to move forward on votes for both measures. The final tally was closer than anticipated because a number of Republicans voted against the rule, requiring eight Democrats to vote yes.

The deal's support comes from the business community -- including the National Association of Manufacturers' President Jay Timmons, who on Thursday called it a "simple decision."

"Vote in support of TPA and expand opportunities for manufacturers to sell our products overseas, increase global competitiveness and fuel our ability to grow and create jobs," he said, "or vote to keep manufacturers on the sidelines and at a global disadvantage."

While that vote has dominated Washington's focus in recent days, most Americans know little about the deal that could be put on the fast legislative track.

Difficult to understand

And even if Americans could review the deal, it wouldn't make much sense. Active trade negotiated texts are blanketed with brackets and extra sentences saying, in a short-form decipherable only to experienced hands, which points, lines and even words are opposed by some countries, or favored by others.

Obama said in a late April interview with The Wall Street Journal that the deal will be available for all to review -- once it's done.

"There are going to be many months in which people will be able to look at every comma and period and semicolon in this deal," he said. "And I feel very confident that when people evaluate the actual deal that is done, that they will see that, in fact, it is the most progressive trade deal in history."

But in the meantime, trade negotiators themselves are barred from sharing the details of the negotiating text. Countries involved in the talks make confidentiality pacts before the negotiations start, knowing that revealing one side's movement before it's clear what that country is getting in exchange can set off the sorts of political alarms that can grind talks to a halt.

Several hundred representatives of major corporations, labor unions and others also get more access than the general public.

They serve on advisory committees created by Congress that get access to U.S. proposals -- though not the full negotiating text, which is summarized for them. Those advisory committees were expanded under the Obama administration to include liberal interests that hadn't been invited to the committees' sessions in the past.

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"The Obama administration has grown the size and membership of the advisory committees that Congress established to add voices that were initially left out of the process, including labor and environmental advocates," said Matthew McAlvanah, a spokesman for U.S. Trade Representative Michael Froman.

"These advisers receive access to U.S. negotiating proposals and work with our negotiators in an interactive process that includes regular updates on the negotiations, the opportunity to review U.S. proposals before they are tabled, and the chance to provide meaningful input into negotiating proposals and decisions," he said.

But those committees are bound by confidentiality agreements, too. That means labor officials like AFL-CIO President Richard Trumka and two of his advisers, including Lee, know what the United States is pushing, but can't tell their members.

"A summary, to be honest, is almost completely useless. It's a summary," Lee said. "If you're used to reading these trade agreements, the devil is absolutely in the details. You want to know: What are the qualifications? What are the exceptions? What's in the actual language?"

Froman's office points out that American trade officials have held more than 1,600 briefings on the deal -- spending countless hours on Capitol Hill going over its details, with Froman personally meeting with Democratic Sen. Elizabeth Warren of Massachusetts, one of the deal's most strident critics.

U.S. negotiating goals have been publicized on the trade representative's blog, which includes in-depth summaries, recaps of negotiating rounds and more.

Congress takes interest

It's only in recent months, though, that Congress has taken increased interest in the negotiations.

From 2012 through March 2015, Froman's office kept the negotiating text of the deal itself and provided briefings to every lawmaker that requested one. In that time period, three senators and 40 House members took them up on the offer.

Once Democratic opposition increased, Froman's office -- at the behest of House Minority Leader Nancy Pelosi -- moved the text into reading rooms at the Capitol, where it's available to lawmakers any time they want to review it.

But members of Congress have howled that the text is so dense, and riddled with jargon, that their trade staffers should be able to leave with it and review it.

The briefings have extended to the private sector, too.

The AFL-CIO's Lee acknowledged that trade negotiators have made an effort.

"No question: There are a lot of hours of briefings that labor has gotten," she said. "But there's a difference between counting the hours and listening to what we've said. They could assign an intern to sit with me for 1,000 hours, but if they didn't take what I said, then it doesn't do me any good.

"Nobody's talking about that. Nobody's keeping a log of the minutes or the hours," Lee added. "Our government doesn't even take what we say and put it on the table."