WASHINGTON — A landmark trade bill that tops President Barack Obama’s second-term agenda faces a showdown vote in the House as Democrats mount a last-ditch effort to kill it.

The outcome was uncertain and the drama intense heading into today’s votes. In frantic 11th-hour maneuvering, liberals in the House defied their own president and turned against a favored program of their own that retrains workers displaced by trade. Killing the program would kill the companion trade bill, and many Democrats and labor leaders advocated just that.

The move caught the GOP off-guard. House Republicans, already in the awkward position of allying themselves with Obama, found themselves being asked by their leaders to vote for a worker retraining program that most have long opposed as wasteful. Many were reluctant to do so, leaving the fate of the entire package up in the air, and Obama facing the prospect of a humiliating defeat at the hands of his own party members — unless he can eke out what all predict would be the narrowest of wins.

“If we have to pass something that’s a Democratic ideal with all Republicans to get the whole thing to go,” said Rep. Tom Rooney, R-Fla., “we could be in trouble.”

The main trade bill at issue would give Obama so-called “fast track” authority to negotiate trade deals that Congress could approve or reject, but not amend. He hopes to use the authority, already agreed to by the Senate, to complete a sweeping pact with 11 other Pacific Rim nations which would constitute the economic centerpiece of his second term. Obama said such a pact with Japan, Mexico, Singapore and other nations constituting 40 percent of the global economy would open up critical new markets for American products.