Nina Easton‘s latest column in Fortune magazine discusses the contradiction between Democrats’ professed support for free trade and their need to placate an important voting bloc.

From President Obama on down, a number of leading Democrats extol free-trade agreements as central to America’s economic growth. There’s only one problem: They like the votes they hope to extract from labor unions in November even more.

With control of the Senate hanging in the balance — and polls showing an enthusiasm gap favoring Republicans — Democrats need the get-out-the-vote muscle of their union allies. And union leaders are predictably using their inside-the-Beltway muscle to block passage of so-called fast-track trade promotion authority, which enables trade deals, delicately negotiated with foreign governments, to travel through Congress on an up-or-down vote without being amended to death.

“‘No’ on fast track … out of the question,” House Democratic leader Nancy Pelosi recently told cheering United Steelworkers. A month earlier, Senate Majority Leader Harry Reid threw cold water on President Obama’s State of the Union endorsement of fast track by saying he would refuse to let it come to a vote.

At risk are two major trade deals being negotiated by the Obama team, one with Asia and the other with the European Union. The Trans-Pacific Partnership — the centerpiece of Obama’s 2011 “pivot” toward Asia — would be the largest U.S. trade deal ever. It would also impose new environmental rules and labor protections that will help level the field for U.S. workers. Across the pond, the Transatlantic Trade and Investment Partnership, TTIP, would remove such trade barriers as tariffs, unnecessary regulations, and restrictions on investment.