Sitting in his office near the White House in mid-July, fresh off a flight from Malaysia, Obama’s chief trade negotiator toed a cautious line. “We’re taking nothing for granted,” said a shirtsleeved Froman. And with good reason. The deal itself is diabolically complex, interweaving nearly $28 trillion in economic activity into a new order that will seek to ease the flow of trucks, pills, beef, and other goods across an ocean while also rewriting rules for everything from copyright protections to labor standards. Selling it to a skeptical American audience presents another challenge altogether.

As U.S. Trade Representative, Froman, 52, should occupy an outer orbit in Obama’s cabinet. But he claims longer-lasting ties to the President than anybody else in the administration—back to Harvard Law, where the two worked on the law review. Froman went on to serve as then-Treasury Secretary Robert Rubin’s chief of staff in the Clinton administration and followed Rubin to Citigroup (C). When Obama first emerged on the national scene, it was Froman who introduced him to the Wall Street donor network, and Obama has kept him close in a variety of roles ever since.

In the White House, Froman started as a chorus of one for an aggressive trade agenda. A trip to Korea with Obama in fall 2010 kicked off a running discussion with the President about the need for the U.S. to forge tighter economic ties in the region, in part to check a rising China. “He’s the intellectual author of the approach,” says Tom Donilon, Obama’s former National Security Adviser. “He saw the strategic implications.” Internally, Froman de-emphasized any immediate jobs boon from a trade deal, understanding that NAFTA’s failure to deliver on that score still haunts Democrats. But as more countries joined the talks, culminating with Japan in 2013, resistance within the administration melted away.

Democrats on Capitol Hill have proved a lot tougher. Twice this spring and summer, they nearly derailed the administration’s push for fast-track authority to finalize negotiations on the pact. Froman’s office has conducted nearly 1,700 Hill briefings—including 200 by Froman personally—and managed to peel off fewer than a fifth of congressional Democrats. But that was enough to keep the process moving: Froman and his team are now crisscrossing hemispheres to iron out the last bumps in the accord, aiming for a late-summer finish. And then, once again, he’ll have to face friendly fire on the Hill. “We’re going to talk to anybody who’s willing to talk to us about the substance,” Froman says. He’s had plenty of practice.

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A version of this article appears in the August 1, 2015 issue of Fortune magazine with the headline “Obama’s Trade Whisperer.”