AFL-CIO President Richard Trumka testifies on Capitol Hill in Washington on April 21, 2015, before a Senate Finance Committee hearing on fast-track trade authority. (Evan Vucci/AP)

The head of the nation's largest labor union is ramping up his rhetoric to fight President Obama's Pacific trade pact, calling a key part of the White House argument "horse waste."

AFL-CIO President Richard L. Trumka has been speaking out against Obama's bid to win approval for a 12-nation free trade deal in the Asia Pacific region for months. In an interview with Washington Post reporters and editors Thursday, Trumka warned that American workers would punish Democrats at the polls if Obama succeeds in getting the Trans Pacific Partnership through Congress.

"If the president wins this fight, Democrats will be in the minority for a decade or so" on Capitol Hill, Trumka said during a hour-long conversation at The Washington Post's offices. If Obama is seeking to build on his legacy with the TPP, Trumka added, "it will be the wrong legacy."

In sometimes colorful and pugilistic language, Trumka also took direct aim at one of the administration's key arguments on the trade bill. Democrats have called for tough provisions aimed at limiting other nations from manipulating their currency to artificially boost exports to the United States.

White House officials, Trumka said, have said such a provision is a non-starter in part because it could expose the United States to punitive actions if the U.S. Treasury enacts quantitative easing to stimulate the economy.

"That’s just pure, unadulterated horse waste," Trumka said.

Yet many independent economists say such ma­nipu­la­tion is no longer prevalent and that the longtime poster child of currency ma­nipu­la­tion — China — has gradually stopped doing it. Federal Reserve Chair Janet Yellin warned Congress not to include currency provisions in the trade deal during congressional testimony in February.

The AFL-CIO has opened a full-fledged assault on Obama's trade push, suspending political contributions to funnel money into an advertising campaign to stop the trade bill from advancing. The Obama administration's "fast-track" authority that would smooth the path to complete the TPP failed to clear a procedural hurdle in the Senate on Tuesday, but Senate leaders have struck a deal to try again over the coming days.

Organized labor has largely opposed liberalizing free trade, blaming past deals such at the North American Free Trade Agreement for killing jobs and lowering wages as U.S. companies sent factories abroad.

Obama has cast the TPP as a new model agreement with higher labor and environmental protections with better enforcement provisions, but he has failed to convince a broad progressive coalition that is fighting against him.

Trumka said that his organization will consider the votes on fast-track authority "a very, very major vote on the scorecard" as union members assess who to support in the 2016 election cycle. And he pledged that the AFL-CIO will be "hard-edged" in going after lawmakers who back the trade agenda.

He also said the AFL-CIO was disappointed with both parties in the midterms last November for failing to articulate a strategy to raise wages for workers.

"What we want to see is someone who has an agenda for working people and will fight for that agenda to make it a reality," he said. "Both parties are on probation after the election." In a meeting with Obama a few months ago, Trumka said, he told the president "that if the first thing that comes out of that is this fight [on trade], people aren’t going to hear anything after that."

Trumka suggested Obama would be better served trying again on comprehensive immigration reform, which died in the GOP-controlled House last summer, or to lobby Congress to increase budgets on infrastructure. However, it is unlikely the Republicans in control of both houses would adopt the president's agenda in either of those areas.

Of Hillary Rodham Clinton, who supported the TPP as Obama's secretary of state but has not taken a position since declaring herself a presidential candidate, Trumka said a day of reckoning is nearing for Clinton and the other 2016 candidates to take a definitive stance.

"I think it's coming now. People are asking her direct questions on the issues," he said. "You can’t run, you can’t hide."