WASHINGTON — Republican and Democratic senators plan to introduce legislation that would force US President Donald Trump to obtain Congress' approval before imposing tariffs on national security grounds, a senior senator said on Tuesday.

Republican Senator Bob Corker, chairman of the Foreign Relations Committee, said legislation would be introduced this week that would pare back the president's authority under Section 232 of the Trade Expansion Act of 1962.

The president decided last month to open a trade investigation into whether auto imports had damaged the US auto industry, which could lead to tariffs of up to 25 percent on "national security grounds".

He had cited similar security concerns in March in imposing duties on steel and aluminum.

Trump, who campaigned on a pledge to negotiate better trade deals to save US jobs, has pursued aggressive measures against trading partners from China to Canada, to Mexico and US allies in Europe.

That has worried some Republican lawmakers who strongly back free trade, warning that Trump could trigger a trade war that would destabilize the economy and ultimately hurt American workers.

"What this would do is redefine that and say that the president would go through the same steps that he goes through, but at the end of the day, if he decides that he wants to put tariffs in place, Congress would have to approve those," Corker told reporters.

On Tuesday, Mexico put tariffs on American products ranging from steel to pork and bourbon, retaliating against the import duties on metals imposed by Trump and taking aim at Republican strongholds ahead of US congressional elections in November.

The tariff list, published in the government's official gazette, included a 20 percent duty on US pork legs and shoulders, apples and potatoes and 20 to 25 percent duties on types of cheeses and bourbon.

Mexico also took steps on Tuesday to make it more attractive for other countries to send it pork by opening a tariff-free quota for some pork imports. Economy Minister Ildefonso Guajardo said his country would now "surely" look to Europe for pork products, used in many traditional dishes in Mexico.

"We need trade, and one of the things we're concerned about is long-term implications that these trade issues will have on our partnerships with Mexico and Canada and other markets," said Iowa Secretary of Agriculture Mike Naig. "If our customers around the world start going to other parts of the world for their supplies, that is a serious problem."

Mexico's response further raises trade tensions and adds a new complication to efforts to renegotiate the trillion-dollar North American Free Trade Agreement between Canada, the United States and Mexico.

White House spokeswoman Sarah Sanders said Trump was still looking at the possibility of doing individual trade deals with Canada and Mexico in place of NAFTA.

"He's looking at the best way to make sure he gets the best deal possible for American workers," Sanders said.

REUTERS