WASHINGTON — President Donald Trump spent more than a year browbeating the leaders of Canada and Mexico into agreeing to a rewrite of North American trade rules. And on Friday, leaders of those two nations are set to sign the pact at the Group of 20 summit in Buenos Aires, Argentina.

Now, Trump faces what could prove a more formidable foe: His own Congress.

Emboldened by their takeover of the House starting next year, many Democrats say they want the new agreement to strengthen its protections for American workers from low-wage Mexican competition. Yet any such changes could raise new objections from Republican free traders who want to limit the ways the pact could restrict corporate practices in North America.

What Trump had hailed as a triumph for his administration — a newly named U.S.-Mexico-Canada Agreement to replace the 24-year-old North American Free Trade Agreement, which he’d long condemned as a job-killer for Americans — now faces a hazier future.

“It’s going to be a very tough sell,” said Democratic Rep. Bill Pascrell of New Jersey, top Democrat on the House subcommittee that oversees trade issues.

Leaders of the three countries agreed to the USMCA just hours before a U.S.-imposed Sept. 30 deadline. Yet the legislatures of the three countries must still ratify it. Many trade analysts say the new NAFTA isn’t very different from the old one despite Trump’s claim that it would “transform North America back into a manufacturing powerhouse.”

“It’s really the original NAFTA,” said Mickey Kantor, a partner at the law firm of Mayer Brown and U.S. trade representative in the Clinton administration.