WASHINGTON (AP) - Debates on lowering trade barriers can turn Congress upside down for Democratic presidents promoting such legislation. Business-minded Republicans suddenly turn into allies and Democrats aligned with organized labor can become outspoken foes.

It's a reversal of the usual order of things, where a Democratic president can generally count on plenty of support from fellow Democrats in Congress along with varying levels of resistance from Republicans.

Now it is President Barack Obama's turn to experience such a role reversal. Already, he is encountering pockets of Democratic resistance, especially from those representing manufacturing states, to his efforts to win congressional approval for renewal of "fast track" negotiating authority.

Such expedited powers help speed the process for major trade agreements by restricting Congress to up-and-down votes on what's already been negotiated - with no amendments allowed.

Two such free-trade deals are in the works. One is a Pacific Rim trade pact - the Trans-Pacific Partnership - between the United States and 11 Asian and Latin American nations. A final round of negotiations begins next month and may be wrapped up by year's end.

"We are now in the endgame," said Acting Deputy U.S. Trade Representative Wendy Cutler.

The other negotiation, not as far along, is a trans-Atlantic trade alliance, mainly between the United States and European Union countries. So far it hasn't generated as much controversy as the nearly done trans-Pacific deal - largely because Europe is a generally high-cost, generally high-wage manufacturing area.

Obtaining fast-track authority from a deeply divided Congress will be a hard sell for Obama, one likely to get even harder as November's midterm congressional elections draw nearer. Democrats now control the Senate, Republicans the House.

And while Obama will likely mention his high-profile trade initiatives in Tuesday's State of the Union address, dwelling on them to any extent could awkwardly bring him more applause from the Republican side of the aisle than from the Democratic side.

Getting a renewal of fast-tack negotiating authority - also called trade promotion authority - "is a priority, not in theory but in fact, for the administration because it is a key part of our overall economic strategy and our foreign policy, particularly in Asia, and because it's time for Congress to update and to assert its own role in trade negotiations," said White House spokesman Jay Carney.

Democratic President Bill Clinton had the tables turned on him two decades ago and found he had to mostly twist Democratic arms to muscle through Congress the North American Free Trade Agreement, or NAFTA - a 1993 free-trade pact among the U.S., Canada and Mexico that Republicans heavily supported. Democratic critics still argue that agreement continues to cost U.S. jobs. Clinton used "fast track" authority in 1993 for winning its passage.

And fast-track rules helped Republican President George W. Bush win passage of trade agreements with Chile, Singapore, Australia and Peru.

Obama used the same tactics to win approval in 2011 of free-trade deals with Colombia, Panama and South Korea, overcoming Democratic opposition.

But that authority has since expired. And opposition is mounting among Democrats to restoring it.

Two House Democrats, Reps. Rosa DeLauro of Connecticut and George Miller of California, sent a letter to Obama last week announcing they had already lined up 151 House Democrats in opposition to what they called "fast-track procedures that usurp Congress' authority over trade matters."

"The United States cannot afford another trade agreement that replicates the mistakes of the past," they wrote on behalf of all the Democratic signers. "We can and must do better."

Organized labor is also digging in its heels. "The AFL-CIO opposes this legislation in the strongest of terms and will actively work to block its passage," said the labor organization's president, Richard Trumka. "It is past time for the United States to get off the corporate hamster wheel on trade."

Right now "we're seeing the same pattern" as before beginning with NAFTA and through subsequent trade-liberalization initiatives, said Alan Tonelson, an economic analyst with the U.S. Business and Industry Council, which represents medium-size companies employing fewer than 500 workers.

Democratic senators, with six-year-terms, have been a little more supportive of fast-track trade legislation than their House counterparts, who serve two-year terms and are often more sensitive to at-the-moment public sentiment. Senate Finance Committee Chairman Max Baucus, D-Mont., introduced the Senate version of the fast-track legislation.

But Baucus is also Obama's pick to be his next ambassador to China and isn't expected to be around to shepherd the bill to passage.

Senate Majority Leader Harry Reid, D-Nev., suggested the Senate isn't about to rush into debate on the measure.

Rep. Dave Camp of Michigan, the Republican chairman of the House Ways and Means Committee and an Obama antagonist on budget issues, is a top Obama ally in the trade debate. But Rep. Sander Levin of Michigan, the top Democrat on the House committee overseeing trade and a strong supporter of Obama on most other economic issues, is a top opponent.

Obama has now taken both sides of the free-trade debate. He opposed such deals while running for president in 2008 - and even criticized Democrat Hillary Rodham Clinton for having supported NAFTA during her husband's presidency.