Unions may be weaker, smaller and more embattled, but they remain crucial players in Democratic politics. When wealthy Republicans began pouring millions into super-PACs in 2010, unions worked to shield Democratic candidates from the onslaught, becoming their biggest source of super-PAC donations.

What is that loyalty worth? With President Barack Obama and Congress negotiating fast-track legislation on international trade, union leaders are about to find out.

Renewal of the president's "fast-track" authority would limit congressional influence over trade pacts negotiated by the administration, protecting trade agreements from being filibustered or amended.

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The conflict over fast track is fundamental. Organized labor wants to kill the legislation. Obama wants to sign it. The rough outlines of the bill would enable Congress to make its preferences known and receive updates while trade negotiations are under way in exchange for a clean vote -- no amendments -- on a final trade agreement.

Even as the White House pushes for fast track, it is negotiating the Trans-Pacific Partnership, an agreement to facilitate trade among the U.S., Australia, Japan and other Pacific Rim nations. Japan insists on fast-track's approval before finalizing a deal.

Labor leaders argue that U.S. workers have lost jobs due to prior trade agreements, including the North American Free Trade Agreement and others with Caribbean nations, Colombia and Korea. Factories and jobs have moved overseas to countries with low wages and minimal environmental standards. Meanwhile corporate profits in recent years have been strong while wages have stagnated. And the number of Americans who say the nation is on the wrong track remains high.

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Business leaders in industries as diverse as technology and agriculture counter that lower tariffs and streamlined customs would enable U.S. companies to increase exports and create more domestic jobs.

Unions including the AFL-CIO, Service Employees International Union and Teamsters last month ratcheted up pressure on Democrats, suspending donations to federal super- PACs"until further notice." Essentially, the unions have opted to sit on the political sidelines until the direction of the legislative fight is clear. The donation freeze affects House, Senate and presidential political committees, including one gearing up to back Hillary Clinton, said one labor official.

Unions donated $166 million between 2010 and 2014 to two Democratic congressional super-PACs. Of $47 million raised by the Senate Majority PAC in 2014, $15 million came from labor unions, making them collectively the committee's second largest donor.

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Unions contributed $11 million to the House Majority PAC's $30 million kitty in 2014, tying the financial industry as top donor.

By contrast, Karl Rove's American Crossroads super-PAC, just one of a half dozen major Republican super-PACs operating in recent elections, collected $176 million between 2010 and 2014. The Republican advantage, which grows significantly when anonymous donations are included, only underscores how important labor's contributions are to Democrats.

If fast track legislation goes through, labor could withhold donations from Senate and House super-PACs. Thus party leaders wouldn't be funneling union money to lawmakers who voted for fast-track authority or TPP. Union leaders could even try to punish wayward Democrats in a Democratic primary fight.

Unions have made similar threats in the past with little result, however. "It's a little bit like Obama and chemical weapons," said former SEIU President Andy Stern. "What you do when someone crosses the lines establishes whether people think these lines are real or not."

Jeanne Cummings writes on money, lobbying and politics.