WASHINGTON — President Trump announced his intention late Saturday to quickly withdraw the United States from the North American Free Trade Agreement, a move intended to force House Democrats to enact a revised version of the pact despite concerns that it fails to protect American workers.
“I will be formally terminating Nafta shortly,” Mr. Trump told reporters aboard Air Force One en route from the Group of 20 conference in Buenos Aires, a day after appearing at a ceremonial signing of the new deal with Canada and Mexico.
If the president follows through on his threat, congressional leaders will have six months to pass the measure. The agreement has been losing support in recent days as Democratic lawmakers, ready to take control of the House in January, reckon with fallout from the announcement last week that General Motors was planning to idle five plants in North America.
If no deal can be reached, both versions of the treaty would be void, which would result in far more restrictive trade that could have a severe impact on industry and agriculture in all three nations, economists have warned.
In other remarks to reporters on the presidential plane, Mr. Trump said he would consider signing a two-week extension of government funding to give lawmakers more time to negotiate a seven-bill spending package, as well as to accommodate memorial services honoring former President George Bush, who died on Friday. Mr. Trump had told lawmakers he would veto any bill that did not include $5 billion for a southern border wall, raising the prospect of a partial government shutdown.
Mr. Trump also told reporters that his second summit meeting with Kim Jong-un, the North Korean leader, could be held in January or February, which he and his aides have suggested before.
The president’s Nafta move is the first indication of how he plans to deal with the new Democratic majority. Mr. Trump seems to have opted for diplomacy at the end of a political barrel, telling reporters on Saturday that Democrats “will have a choice” of whether to approve the deal as written or risk the consequences.
Even as he suggested he would make such a drastic move, he played down its potential impact, adding that he would have no problem reverting to a “pre-Nafta” environment. Such a scenario “works very well” for the United States, he said.
Nancy Pelosi, the House Democratic leader who is likely to be elected speaker, cast doubt on the likelihood that the deal could be passed without significant new assurances from Mexico that labor standards in the agreement will be strictly enforced.
Ms. Pelosi described the deal on Friday as a “work in progress” her members could not yet support. “What isn’t in it yet is enough enforcement reassurances regarding workers, provisions that relate to workers and to the environment,” she said.
A spokesman for Mrs. Pelosi did not say on Sunday if the president’s move would accelerate her timetable or alter her negotiating strategy. “It’s disappointing but not surprising that President Trump would try to force Congress to reinstate the status quo of Nafta, instead of working constructively with Congress to improve his proposed agreement to actually protect and strengthen American workers,” said the spokesman, Henry Connelly.
There is no language in Nafta’s authorizing law that requires congressional approval for withdrawal from the treaty, although some of Mr. Trump’s staff believe the matter is ambiguous and could end up in court.
Congress, at Mrs. Pelosi’s instigation, could proactively pass legislation, with a veto-proof majority, blocking him from doing pulling out of Nafta. But most legislative leaders think that scenario is unlikely.
Mr. Trump’s announcement, which came shortly after he agreed to hold off on new tariffs against China in face-to-face negotiations with the Chinese leader Xi Jinping, was also intended to emphasize his tough stance on trade as he softens his posture with Beijing.
The move took many of Mr. Trump’s economic advisers by surprise: In the lead-up to the Argentina trip, most of them were said to believe that scrapping Nafta was off the table.
But Mr. Trump has grown increasingly frustrated with Democrats, telling people in his orbit that he believes they would rather turn their backs on a “great deal” than see him achieve one of his top campaign goals, according to a person who has spoken to the president in the past week.
Peter Navarro, Mr. Trump’s hard-line trade adviser, had long prodded the president to formally withdraw from the old Nafta deal, and Mr. Trump has frequently threatened to do so during meetings with his staff members and foreign leaders.
Until Friday, he had been talked out of making such a move by moderate advisers including Gary D. Cohn, the former head of the National Economic Council; his successor, Larry Kudlow; and Treasury Secretary Steven Mnuchin, who scored a significant victory on Saturday after pressing the president to hold off on the new tariffs against China.