WASHINGTON -- Sen. Elizabeth Warren (D-Mass.) has been the public face of the Democratic Party 's feud with President Barack Obama over his trade agenda. But behind the scenes, Sen. Sherrod Brown (D-Ohio) quietly united his party behind a strategy that resulted in a major defeat Tuesday for the president.

Brown's weeks of work came to fruition when Democrats voted to block legislation that would have given Obama so-called fast-track trade authority. Fast-track authority would strip Congress of the ability to amend trade deals negotiated by the president and is essential for the passage of Obama's Trans-Pacific Partnership, a trade deal the administration is negotiating with 11 Pacific nations.

Browns opposition to giving Obama expedited powers to funnel a trade deal through Congress is no surprise, but his hand in uniting Democrats, specifically those supporting fast-track, proved pivotal on Tuesday.

Democrats held up the fast-track legislation in the Senate Finance Committee for months, until Sen. Ron Wyden (D-Ore.) cut a deal with Obama and Sen. Orrin Hatch (R-Utah), securing a handful of Democratic concessions. But Hatch didn't package those concessions into the fast-track bill itself. Instead, they were included in other pieces of legislation that were considered at the same markup hearing. Trade Adjustment Assistance, a program that provides job training and financial aid for workers who lose their jobs to foreign trade, was presented as its own bill, as was a customs enforcement bill.

Hatch allowed a handful of other Democratic amendments to sail through in the enforcement bill. The two most important items cleared with bipartisan support. Both were Brown projects that would significantly alter TPP and the enforcement of other existing trade deals.

To get Senate leadership on board, Brown immediately reached out to Senate Minority Leader Harry Reid (D-Nev.) following the Finance Committees markup of those bills, arguing they should be voted on as a package. "Hatch was already making noises that he would pull out TAA and pull out customs," Brown told reporters after Tuesday's vote, referring to individual provisions. Reid took it from there, threatening, in an interview with The Huffington Post on May 4 , that Democrats would block moving to a fast-track bill unless all four measures were considered together.

It seemed to be coming together right from the start, from the moment I talked to Senator Reid about it, Brown said.

Ahead of Tuesdays vote, Brown said he made some 25 calls to members of his caucus, urging them to vote against bringing the fast-track bill to the chamber floor because it did not include commitments on enforcement.

I talked to people particularly who were pro-TPA, Brown said. Some people [I called] two or three times, mostly talking to people who were already for TPA, some that were undecided.

One Democratic aide told The Huffington Post that Brown gave a pretty impassioned plea in caucus and has been calling his colleagues over the past few weeks to encourage them. The aide added Brown had many one-on-one conversations with leadership.

Senate Minority Whip Dick Durbin (D-Ill.) confirmed Browns role in rallying Democrats behind a strategy that called for a vote on all four trade bills.

Sen. Brown brought it up to me right away," Durbin said shortly before the vote. "He thought keeping them as a package was critical and he worked the caucus on it."

Brown was the only senator to sign a letter with Warren firing back at Obama, after the president accused her and other trade critics of being "dishonest" about the trade debate.

Brown scolded Obama for his comments toward Warren, calling it disrespectful by referring to her as her first name, when he might not have done that for a male senator, perhaps?"

Brown said its now up to Senate Majority Leader Mitch McConnell (R-Ky.) to decide whether he will bring the bill up again.

If McConnell thinks it's killed, it's killed," Brown said. "If McConnell wants to bring it back with all four provisions, it would be on the floor today and we'd be debating it and the amendment process would begin.

McConnell indicated earlier on Tuesday that he was willing to loop Trade Adjustment Assistance to fast-track, but not the customs enforcement bill.

The Senate fight, however, is just a warm-up for the House battle, which promises to be even more difficult for backers of the trade bill. In the House, tea party Republicans are deeply skeptical of the trade deals, with many grassroots activists considering TPP to be a step toward a one-world government.

"The House doesn't have the votes to pass any one of these individually," Brown said, except a small one that relates to Africa and Haiti. "They've got to figure out how they hold the four together [in the House]. It's not my job to figure out how to pass something in the House."

For his part, McConnell said he remained committed to processing TPA and TAA, and other policies Chairman Hatch and Senator Wyden can agree to after Tuesdays vote. But blocking the Senate from even debating such an important issue is not the answer, he added.

Still, Democrats want more, including an amendment offered by Brown and Wyden during the committee markup that would ban products made with forced labor from entering the U.S. It's a straightforward human rights project that received nine votes each from Republicans and Democrats. But three countries currently involved in TPP talks -- Vietnam, Malaysia and Brunei -- are forced-labor hotbeds.

While the Obama administration has persistently argued that TPP will include robust labor protections, most Democrats remain skeptical that those standards will be enforced. The administration has a poor record of enforcing labor rules in existing trade deals , but the Brown-Wyden amendment wouldn't depend on trade staffers sorting out foreign worker abuses before international panels. It would direct domestic law enforcement to send back any products made with forced labor. While that would almost certainly be permitted under TPP, it would reduce the value of the deal for Vietnam and Malaysia.

The other amendment, penned by Brown and Sen. Chuck Schumer (D-N.Y.), would combat currency manipulation by foreign governments, a top priority for Democrats. By devaluing their currency, governments can make their own goods cheaper overseas without lowering standards of living domestically. While China is the most notorious currency manipulator, Japan and other countries involved in the TPP talks have been almost as aggressive.

The Schumer-Brown amendment would require the U.S. Department of Commerce to include the effects of currency manipulation in considering whether to bring a trade case against a foreign country. It isn't limited to TPP, and could apply to cases that American firms want to bring against China.