Carmakers also would be required to use more local steel, aluminum, glass and other parts. And 40 to 45 percent of vehicles would have to be made by workers earning at least $16 an hour. That increase is intended to increase jobs in the United States and Canada, where wages are higher.

Those changes are generally acceptable to Canada, but it remains resistant to broadening access to the Canadian dairy market and to abandoning an independent tariff dispute settlement system. Mexico has agreed to put off discussions over removing tariffs that Mr. Trump has imposed on steel and aluminum imports, but Canadian negotiators have argued that these should be lifted as part of a new Nafta deal.

Robert E. Lighthizer, the Trump administration’s chief negotiator, told lawmakers this week that talks with Canada would continue even after the release of the agreement with Mexico. Administration officials are hopeful that Canada will be more willing to make concessions after provincial elections take place in October. Modest changes to the text can be made after it has been released and even after it has been signed.

“Going forward, Ways and Means and Finance Republicans and Democrats will be analyzing this carefully and looking for ways to advance a three-country agreement,” Representative Kevin Brady of Texas, the Republican chairman of the House Ways and Means Committee, said on Friday.

He added: "I’m hopeful Canada steps it up and steps to the table in a big way.”

Business groups have also been expressing their trepidation about moving forward with a free-trade agreement that does not include Canada.

“If Canada doesn’t come into the deal, there is no deal,” Thomas Donohue, the president of the U.S. Chamber of Commerce, warned last week. Those concerns were echoed on Friday by the Business Roundtable, a lobbying group that represents the leaders of major American companies. “Our northern ally is critical to allow trade to strengthen the U.S. economy,” the group said.

Relations between the governments of the United States and Canada have fallen to their lowest point in recent memory, leaving open the possibility that no deal will be reached anytime soon. Mr. Trump said this week that he was frustrated with how Canada had been behaving during negotiations and vowed that he would be just as happy to levy tariffs on imports of Canadian cars — a move that would likely spur backlash from carmakers and consumers in the United States.