WASHINGTON — House Democrats rebuffed a dramatic personal appeal from President Obama on Friday, torpedoing his ambitious push to expand his trade negotiating power — and, quite likely, his chance to secure a legacy-defining trade accord spanning the Pacific Ocean.

In a remarkable rejection of a president they have resolutely backed, House Democrats voted to kill assistance to workers displaced by global trade, a program their party created and has stood by for four decades. By doing so, they brought down legislation granting the president trade promotion authority — the power to negotiate trade deals that cannot be amended or filibustered by Congress — before it could even come to a final vote.

“We want a better deal for America’s workers,” said Representative Nancy Pelosi of California, the House minority leader who has guided the president’s agenda for two terms and was personally lobbied by Mr. Obama until the last minute.

Republican leaders tried to muster support from their own party for trade adjustment assistance, a program they have long derided as an ineffective waste of money and sop to organized labor. But not enough Republicans were willing to save the program.

Republican leaders then passed a stand-alone trade promotion bill, but that would force the Senate to take up a trade bill all over again. And without trade adjustment assistance alongside it, passing trade promotion authority in the Senate would be highly doubtful.

The vote was an extraordinary blow to Mr. Obama, who went to the Capitol on Friday morning to plead personally with Democrats to “play it straight” — to oppose trade promotion if they must but not to kill trade assistance, a move he cast as cynical. On Thursday night, he had made an unscheduled trip to the annual congressional baseball game to try to persuade Ms. Pelosi.