“That is a point that we are making very clearly to our U.S. partners and allies,” she said.

Margaritas Schinas, the senior spokesman for the European Commission, said there was “no justification” for the United States to impose tariffs on steel and aluminum on national security, grounds. “Invoking national security would be even more far-fetched in the case of the car industry,” he said.

In an interview Thursday on CNBC, Commerce Secretary Wilbur Ross, who is leading the investigation, said the administration was defining national security “broadly,” to include the impact on employment and other factors. “Economic security is military security,” he said. “And without economic security, you can’t have military security.”

Mr. Trump has framed the threat of tariffs as a potential source of leverage in talks to rewrite the North American Free Trade Agreement, which have largely stalled over rules governing how carmakers can qualify for Nafta’s zero tariffs.

On Wednesday, President Trump said automakers would be “very happy” with his announcement. But few seemed pleased.

While many companies said they were still waiting to see the likely effect of the measure, carmakers, auto parts suppliers and car dealers said it threatened to raise their costs, depress auto sales and jobs in the United States, and make American-made products less competitive worldwide. The industry has already been hit by the tariffs on steel and aluminum, which have weighed on their profit margins, and the threat of looming trade action against China, as well as potential Chinese retaliation.

“If the tariffs actually went into effect, it would upend the supplier industry,” said Ann Wilson, senior vice president for government affairs at Motor & Equipment Manufacturers Association, which represents auto parts makers. “We are very dependent on the ability to bring in parts from other parts of the world for final production in the U.S., and if that’s no longer financially viable, the question becomes, is the production of vehicles in this country going to be financially viable?”

Kristin Dziczek, the vice president of Industry, Labor & Economics at the Center for Automotive Research, said automakers would be slow to adjust their own investment decisions until they see how any potential tariff decisions might come to pass.