President Donald Trump ordered his Commerce Department to consider a probe into whether car and truck imports threaten U.S. national security, a move that could lead the U.S. to slap fresh tariffs on foreign vehicles.

“Core industries such as automobiles and automotive parts are critical to our strength as a Nation,” he said in a statement late Wednesday. An investigation would unfold under the seldom-used section 232 of the 1962 Trade Expansion Act, the same clause the U.S. invoked in imposing global tariffs on steel and aluminum.

The move may further inflame tensions with America’s biggest trading partners, adding to a series of U.S. threats that have roiled financial markets and upset traditional allies. It comes as Republican lawmakers prepare for midterm elections in November that will determine whether the party retains its majority in both the House of Representatives and Senate.

Trump has made protecting American manufacturing workers -- and the iconic auto industry, in particular -- a keystone of his administration. Wins in manufacturing states such as Michigan and Ohio were key to his victory in 2016.

Mexico exports the most passenger cars to the U.S., followed by Canada, Japan, Germany and South Korea, according to U.S. data. Industry observers saw this latest move as a U.S. tactic to pressure Mexico and Canada to move quickly to agree to an overhaul the the North American Free Trade Agreement. Rules for regional content in cars have been one of the thorniest issues in Nafta discussions.

Almost one-quarter of autos sold in the U.S. are imported, according to government figures. The U.S. already levies a 2.5 per cent duty on imported passenger cars and a 25 per cent tariff on pickup trucks from countries that are not parties to free trade agreements.

The Trump administration is already embroiled in trade disputes on a number of fronts.

The U.S. is in talks with China on a deal to avoid tit-for-tat tariffs between the world’s two biggest economies. In a tweet Wednesday morning, the president said a trade agreementwith China may be “too hard to get done” and probably will require a “different structure.” Trump’s remarks damped expectations that the two sides may have reached a truce, afterTreasury Secretary Steven Mnuchin said the U.S. had put its tariff plans "on hold."

The president signaled earlier Wednesday on Twitter that an important announcement was imminent to help the U.S. car-manufacturing industry. “There will be big news coming soon for our great American Autoworkers. After many decades of losing your jobs to other countries, you have waited long enough!” Trump said in the tweet.

Section 232 gives the U.S. president the power to impose tariffs on imports that imperil national security. It has been used sparingly by previous administrations. Earlier this week, China announced it was reducing its own car import tariffs to 15 per cent from 25 per cent.

Since the 2016 election campaign, Trump has repeatedly threatened to slap new tariffs on imported cars.

He revisited the possibility again on May 11, when he floated the idea of a 20 percent tariff on imported autos during a White House meeting with senior executives of 10 majorautomakers, a person familiar with the meeting said. He also suggested that imports should have to achieve tougher emissions standards than vehicles assembled in the U.S., the person said.

But it may be tougher for the U.S. to make a national-security case with a consumer product such as cars, than it did with steel and aluminum, two materials used in military equipment.