WASHINGTON — Hours after President Obama made a dramatic, personal appeal for support, House Democrats on Friday thwarted his push to expand trade negotiating power — and quite likely his chance to secure a legacy-defining accord spanning the Pacific Ocean.

In a remarkable blow to a president they have backed so resolutely, House Democrats voted to end assistance to workers displaced by global trade, a program their party created and has supported for four decades. That move effectively scuttled legislation granting the president trade promotion authority — the power to negotiate trade deals that cannot be amended or filibustered by Congress.

“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.

The vote that prevented the president from obtaining trade promotional authority now imperils the more sweeping Trans-Pacific Partnership, a proposed trade agreement with 11 other nations along the Pacific Ocean that affects 40 percent of the global economy on goods ranging from running shoes to computers.