Jamieson Greer, President Trump’s nominee to be the next U.S. trade representative, defended the president’s plan to impose tariffs on all imported products and told senators he would work to restructure international trading relationships during his confirmation hearing on Thursday.

Mr. Greer, a trade lawyer and former Trump administration official, told the Senate Finance Committee that he believed the United States “should be a country of producers” as well as consumers, and that he would work to open international markets for U.S. farmers and try to “reverse” the deindustrialization of the nation.

Mr. Greer said Mr. Trump’s idea of imposing a universal tariff on all imports should be studied to see if it would reduce the U.S. trade deficit. He also said he would review whether China, along with Mexico and Canada, were complying trade deals they reached with the United States during Mr. Trump’s first term.

“I am convinced that we have a relatively short window of time to restructure the international trading system to better serve U.S. interests,” Mr. Greer said.

Mr. Greer is the former deputy of Robert E. Lighthizer, the trade representative in Mr. Trump’s first term. Mr. Greer negotiated with China, Canada, Mexico and other countries in that role, and supporters say he has an extensive knowledge of trade law.

His position could be an important one, given Mr. Trump’s proposals to upend global trading relationships with sweeping tariffs. The trade representative will likely help carry out Mr. Trump’s tariff wars, and potentially renegotiate the trade agreement the United States has signed with Canada and Mexico.

But it is unclear exactly how much autonomy the position will have in the current administration, given that Mr. Trump has said he will put Howard Lutnick, his pick for commerce secretary, in charge of his trade policy. Mr. Trump himself also has strong views on trade and once remarked that if elected, he would be his own U.S.T.R.

On Thursday, Senate Democrats questioned that arrangement and denounced Mr. Trump’s trade moves over the past week, in which the president came within hours of imposing a 25 percent tariff on goods from America’s largest trading partners, Canada and Mexico, as he sought concessions related to the border and drugs.

Mr. Trump ultimately chose to pause tariffs on Canada and Mexico, but put an additional 10 percent tariff on all imports from China — more than $400 billion of products — on Tuesday.

Senate Democrats said they would work with the Trump administration to combat unfair trade and open foreign markets, but called the actions against Canada and Mexico “reckless” and “erratic.”

Senator Ron Wyden, Democrat of Oregon and the ranking member of the Senate Finance Committee, said he was concerned about other Trump officials using trade policy to pursue goals that had nothing to do with trade.

“International trade policy is just too important to American families, workers, small businesses, manufacturers and farmers to sacrifice to make headlines on unrelated issues,” he said.

Senator Catherine Cortez Masto, a Democrat of Nevada, said a small business in her state had a Canadian customercancel a project because of the uncertain trade relationship, costing them tens of thousands of dollars.

“There’s got to be answers for so many businesses that are actually being, unfortunately, victims of this trade war,” Ms. Cortez Masto said.

Republican senators expressed support for Mr. Greer and the president’s actions. But some expressed concern about tariffs increasing input prices for farmers, and retaliation from other countries affecting U.S. exporters.

Clete Willems, a partner at Akin Gump who worked on trade policy in the first Trump White House, said that Mr. Greer had similar views on trade as those of his former boss, Mr. Lighthizer: that the global trading system had evolved in a way that has not been beneficial or fair to the United States.

Mr. Lighthizer tried “to upend that order and remake a lot of the international trading rules in a way that was more equitable to the United States,” Mr. Willems said. “And I think Jamieson will continue that.”

Mr. Willems said that Mr. Greer would bring to the role years of practical experience and an intimate knowledge of what actions trade laws allow.

“He is going to be at the center of this because they need him,” Mr. Willems said.

The U.S. trade representative leads a small agency of more than 200 people that has offices in Washington, Geneva and Brussels. The office is charged with carrying out trade negotiations and resolving economic disputes with other countries, as well as working with lawmakers, farmers and business owners to shape trade policy.

“Jamieson will focus the Office of the U.S. Trade Representative on reining in the Country’s massive Trade Deficit, defending American Manufacturing, Agriculture, and Services, and opening up Export Markets everywhere,” Mr. Trump said in a statement on social media in November.

In contrast to some of Mr. Trump’s other nominees, who have a more antagonistic relationship with their own bureaucracy, Mr. Greer is liked by many of the staff he will be in charge of, current and former U.S.T.R. employees say.

Before his work at the trade representative’s office, Mr. Greer was a lawyer for the Air Force and was deployed to Iraq. Mr. Greer said Thursday that he had grown up in a mobile home with parents who regularly worked several jobs, and that he was “mindful of the struggles that Americans face when they’re cut out of economic growth.”

After the first Trump term, Mr. Greer became a partner in international trade at the law firm King & Spalding.

His financial disclosures showed that he worked for clients including steel firm Cleveland-Cliffs, agricultural organizations like the J.R. Simplot Company and the National Milk Producers Federation, oil and gas company Talos Energy, and a variety of chemical companies, including BASF. He has promised to resign his position at King & Spalding if confirmed.