As another round of negotiations to review a North American free-trade deal wraps up in Washington this week, lawmakers are growing more vocal in defense of the deal, and anxious about its future.

President Trump has said he’s not sure whether he will renew the six-year-old United States-Mexico-Canada Agreement. While many view the president’s threats as a negotiating tactic, they are still causing concern among members of Congress whose districts depend on the agreement.

The United States and Mexico are negotiating in earnest, trading proposals in Washington this week about cars, agriculture and other topics. But Canada did not join the negotiations, raising questions about how any changes would be translated to a three-party agreement. The Trump administration has at times threatened to jettison Canada entirely.

Representative Linda T. Sánchez of California, the top Democrat on a House trade panel, said in an interview that she was a “big proponent” of maintaining the trilateral structure of U.S.M.C.A.

“I’m concerned that the Trump administration is going to abandon that and do these bilateral, like M.O.U.s, which don’t have the full effect of Congress,” she said, comparing Mr. Trump’s trade deals to informal memorandums of understanding.

Mr. Trump negotiated U.S.M.C.A. in his first term to revise the North American Free Trade Agreement, which he called one of the worst deals ever. U.S.M.C.A. wrapped in priorities of both Republicans and Democrats, and its “implementation act” passed Congress with overwhelming bipartisan support in 2020.

Since then, Mr. Trump and his advisers have grown more critical of the pact, seeing it as responsible for trade deficits with Canada and Mexico, America’s biggest trading partners.

Under the terms of the agreement, the three countries promised to review the deal by July 1 of this year. During negotiations in Mexico last month, the United States proposed changes to the pact’s rules for metals, cars and other goods, including raising the requirement for how much local materials cars need to be made with to be exempt from tariffs.

U.S.M.C.A. requires cars to source 75 percent of their content by value from North America to qualify for tariff-free treatment. The Trump administration would raise that threshold to 82 percent, while requiring 50 percent of a car’s content to come from the United States, people familiar with the proposals said. The United States is also proposing expanding those requirements to new types of car parts and setting new content requirements for other industries, including electronics.

In an interview on Thursday, Jamieson Greer, the U.S. trade representative, said talks this week with Mexican officials had been positive, and included discussions of how to revive the global electronics supply chain in the United States and Mexico, while keeping the U.S. trade deficit with Mexico down.

“We certainly want more U.S. content, but to the extent we’re going to be importing these other things, we want to have them as close to home as possible,” he said of the electronics industry.

Mr. Greer has talked about the importance of keeping parts of U.S.M.C.A. intact, but the president has lately seemed far more critical. In France on Wednesday, Mr. Trump said the United States would “do better without” the pact.

“I would rather not have the agreement, but I may sign it,” Mr. Trump said. “We do better as a country if we don’t have an agreement.”

U.S.M.C.A. and its predecessor, NAFTA, have always been somewhat divisive politically, but the three countries are carrying out negotiations over U.S.M.C.A.’s future at a sensitive political moment, as midterms loom and Mr. Trump’s economy is on the ballot.

While some parties, like the United Automobile Workers union or Florida tomato growers, have intense criticisms of the pact, it has many defenders. The deal’s future is particularly important for states that export farm goods, as well as those on U.S. borders that are economically integrated with Canada or Mexico.

In a hearing this month held by the House Committee on Agriculture, Representative Glenn Thompson of Pennsylvania, the committee’s Republican chairman, said the agreement had proved “extremely beneficial” to farmers, ranchers, consumers “and the economy as a whole.” But he conceded that it had provisions that could be improved.

Democrats are urging changes that they say would benefit workers and the environment. Ms. Sánchez led nearly 20 Democrats in a letter urging Mr. Greer to negotiate provisions that would target threats to economic security, strengthen workers’ rights and add new environmental protections.

Ms. Sánchez’s district is particularly affected by trade because it is close to the Port of Los Angeles. After a public forum in a Los Angeles suburb, a small-business owner told her that he had to close his business because of Mr. Trump’s tariffs. She said she was unsatisfied with the administration’s engagement on U.S.M.C.A., and she blamed her Republican colleagues for ceding power to the president on trade.

“The administration doesn’t seem particularly interested in involving Congress in these trade negotiations,” Ms. Sánchez said.

Republicans have an even trickier calculus. Many come from states that are major exporters, but they also face potential political peril for standing up to the president’s trade agenda. Since Mr. Trump’s return to office, Republicans in Congress have repeatedly moved to try to block votes challenging tariffs.

Many Republicans have tried to walk a line of voicing support for U.S.M.C.A. while saying it has room for improvements.

“A renewed U.S.M.C.A. really, I think, can hold our neighbors accountable, and then work for our own ag producers and our manufacturers,” Representative Adrian Smith of Nebraska, the top Republican on a House trade panel, said in an interview. “Right now, our role is to elevate the profile of the policy and the issue itself, given its importance to our economy,” he added.

Mr. Smith said he was “not a fan of tariffs” but had come around to some of the Trump administration’s efforts to open up foreign markets to American business. His constituents’ reactions to Mr. Trump’s tariffs are “mixed,” he said.

Sidelined from the negotiations, Republican and Democratic lawmakers have penned letter after letter, held briefings with experts and traveled to Mexico City during talks last month to make their priorities heard.

The administration is also sparring with some lawmakers about the role Congress will play in the pact’s approval. Mr. Greer said Thursday that he would seek Congress’s approval if the revised pact made changes to U.S. law, but that the changes were more likely to affect Canada and Mexico than the United States.

“We don’t really have to change our laws to get better terms of trade with Canada and Mexico,” he said. “We need Canada and Mexico to change the way that they’re treating our country.”

The Constitution delegates trade as the legislative branch’s responsibility. But the bill that Congress passed to enact U.S.M.C.A. gives the executive branch the power to modify certain rules, though some say the changes the administration is pushing for go beyond the scope Congress intended.

Mr. Smith said he could imagine “a path that would mean that Congress wouldn’t necessarily need to vote on it again.” But Democrats, including Ms. Sánchez, say Mr. Trump should submit the agreement for a congressional vote.

Lawmakers of both parties have also argued that, even if the administration doesn’t need them to approve its changes, Mr. Trump should turn to Congress if he wants his policies to be long-lasting. Otherwise, the next president could unwind them.

Greta M. Peisch, a trade lawyer who served in the Office of the U.S. Trade Representative under the Biden administration, said there was a “bright line” legally dictating that Congress must have a role when U.S. law was changed or if the United States entered into a new trade agreement.

But if U.S.M.C.A. is modified without requiring changes to the law, it isn’t precisely clear what Congress’s role ought to be. That, she said, is “a really big gray area.”

Tony Romm contributed reporting.