Sanders has been considering a presidential run for months. Like Sen. Elizabeth Warren (D-Mass.), another progressive favorite often mentioned as a potential 2016 contender, Sanders has been critical of the Trans-Pacific Partnership, a major U.S. pact being negotiated with 11 Pacific nations, which Sanders says will exacerbate income inequality and erode important regulations.

The letter Sanders sent on Monday to U.S. Trade Representative Michael Froman criticizes secrecy standards that Froman’s office imposes on the TPP talks.

“It is incomprehensible to me that the leaders of major corporate interests who stand to gain enormous financial benefits from this agreement are actively involved in the writing of the TPP while, at the same time, the elected officials of this country, representing the American people, have little or no knowledge as to what is in it,” Sanders added.

Members of Congress are allowed to view negotiation texts, but their staffers can only see them if they have a security clearance and serve on either the Senate Finance Committee or the House Ways and Means Committee. Sanders does not. Members from both chambers have expressed frustration that they aren’t allowed to keep copies of the proposed text, preventing them from assigning staffers to research or analyze key sections.

Corporate officials have access to negotiation documents through USTR advisory committees, where they significantly outnumber representatives of organized labor, environmental advocates and academic experts. The U.S. Chamber of Commerce — the top lobbying group for corporate America and a consistent backer of Republican politicians — is a strong TPP supporter.

USTR told HuffPost it had ramped up congressional outreach, and has held nearly 1,600 meetings on Capitol Hill over the trade pact.

“Senator Sanders, like all Members of Congress, has full access to the draft TPP negotiating text and we look forward to working with him to review it,” USTR spokesman Trevor Kincaid said. “Members of Congress, labor unions, non-profits, and environmentalists have all played an important role in shaping our approach to our trade policy. This includes Senator Sanders, whose input USTR has received on a dozen occasions on issues ranging from clean energy manufacturing to cheese.”

In his letter, Sanders requests a copy of the draft text that he and his staff can analyze. If Froman, the top USTR official, turns down his request, Sanders said he’d respond by introducing a bill that would require any trade talks be made public at the request of a member of Congress.

But it’s the tone of the letter that sends the strongest message, particularly to a Democratic administration. Sanders calls TPP secrecy “incomprehensible” and “simply unacceptable.” He quotes the Constitution, suggesting that USTR is overstepping its authority. The letter is essentially a threat to antagonize Froman with the legislative process if he doesn’t comply with Sanders’ request. It also is a clear declaration that if Sanders runs for president, trade will be a major part of his campaign platform.

Trade hasn’t occupied the national spotlight since the Clinton years — a phenomenon that elevates its potency in a Democratic primary likely to include Hillary Clinton, who was involved in TPP talks while secretary of state under Obama. And President Bill Clinton’s economic legacy is at the heart of the only major intra-party rift currently dividing Democrats: What to do about Wall Street. The Clinton presidency signaled the Democratic Party’s full embrace of the financial elite, reversing the tough-on-banks platform implemented by Franklin D. Roosevelt under the New Deal. To Clinton’s liberal critics, he had converted to GOP orthodoxy on economic matters.

Clinton’s bank deregulation hasn’t been popular in the years following the 2008 financial meltdown. But many congressional Democrats have been willing to follow the Clinton model of making nice with Wall Street to get the campaign cash needed to advance other liberal goals. The tension spilled into public view last month during a bitter feud over subsidies for risky Wall Street derivatives trades that clearly surprised the Democratic leadership in the House and Senate.

Casting the Clinton-era North American Free Trade Agreement and World Trade Organization treaties, and Obama’s TPP, as similar gifts to corporate insiders would be politically potent in a Democratic primary. Warren has said repeatedly that she isn’t running for president, but her own attacks on TPP have taken precisely this approach. In December, Warren sent a letter of her own to Froman, asking him to prevent foreign corporations from using TPP to challenge domestic laws and regulations before an international tribunal. Granting companies this political power, she said, posed significant risks for financial regulation.

Hillary Clinton, of course, hasn’t announced a presidential bid (yet), and she has stayed generally quiet about trade policy. She praised NAFTA in her 2003 memoir and as a senator from New York, but criticized it on the campaign trail in 2008.

Obama is already giving Sanders and other progressive Democrats fuel for the corporate giveaway narrative. After the Republican rout in the 2014 elections, both Obama and then-Senate Minority Leader Mitch McConnell (R-Ky.) vowed to work together to push through trade deals that liberal Democrats had blocked (although McConnell will have to flout his own burgeoning populist flank to do so).

Plenty of Democrats in Congress support NAFTA-style free trade pacts, arguing that they encourage economic growth and make goods cheaper for U.S. consumers. But with then-Senate Majority Leader Harry Reid (D-Nev.) and the overwhelming majority of House Democrats opposed to TPP, the pact’s Democratic supporters kept quiet in the face of what appeared to be not just a bruising battle, but a losing one. Those calculations have changed since the GOP won control of the Senate and Obama announced his plans to push hard on the deal. The attacks from Sanders and Warren, however, indicate that the fight is going to be intense, whatever the result.

“Sixteen hundred meetings is great,” said one Democratic congressional aide. “So is one copy of the deal.”

Read Sanders’ full letter here.