Larry Cohen, the labor leader and outgoing president of the Communications Workers of America (CWA), is officially backing Bernie Sanders' 2016 presidential campaign—at least in part thanks to Hillary Clinton.

Cohen will reportedly serve as an unpaid volunteer for the U.S. senator from Vermont, who is running on a platform of progressive issues like workers' rights and campaign finance reform, among others.

Cohen told the Huffington Post on Wednesday that he made his choice after Clinton, who is currently the frontrunner for the Democratic nomination, continued to evade questions from the media and consumer watchdogs over her stance on Trade Promotion Authority, also known as Fast Track—a bill that gives President Barack Obama expanded power to push pro-corporate agreements like the Trans-Pacific Partnership (TPP) through Congress without input or amendments from lawmakers.

Fast Track passed Congress last week. Obama signed the legislation into law Monday.

"I did everything I knew how to do to get Clinton to speak out on fast track, and she wouldn’t," Cohen told HuffPo. "We begged her to speak out."

HuffPo continues: "Bernie is movement-building, and we need a new movement."

—Larry Sanders, CWA

Cohen, a staunch progressive, said Clinton's handling of the trade issue helped clarify why he wanted to get behind Sanders, whose candidacy is still considered a long shot despite standing-room-only crowds he's been drawing early on the trail. Sanders has been a vocal critic of giving Obama fast-track trade authority. “Without a candidate like Bernie, we’re going to get a repeat of the same stuff," Cohen said. "Bernie is movement-building, and we need a new movement. We need to get big money out of politics."

While the CWA has not publicly endorsed a candidate for president, the union came out strong against Fast Track and the TPP. In April, before the legislation passed Congress, Cohen called on lawmakers to "put the brakes on Fast Track."

Cohen said: "TPA pretends to be about trade, but in reality it is about protecting corporate profits above all else and defining our national security in terms of giving away our jobs, depressing our wages and then rewarding the responsible multinational corporations, often U.S. based, with guaranteed profits in the nations where they invest."

The labor leader will officially announce his support of Sanders at a campaign stop in Council Bluffs, Iowa later this week.

"The key is him being the progressive candidate," Cohen told HuffPo. "You build the movement—you don't just inherit it from labor or any other tent."

And Clinton's hedging on Fast Track "won't be forgotten," Cohen added. Organized labor is "not a rubber stamp for the Democratic Party and certainly not for corporate Democrats."