The threat against the EU comes after Trump directed the Commerce Department last month to launch an investigation into whether imports of foreign autos were a risk to U.S. national security and should be subject to tariffs or other trade restrictions.

Trump‘s tweet also comes the same day the EU imposed tariffs on U.S. products, ranging from Kentucky bourbon to Harley-Davidson motorcycles, in retaliation against U.S. tariffs on imports of European steel and aluminum.

The U.S. already taxes imports of European cars at a rate of 2.5 percent. Trump has often complained about the 10 percent tariff the EU imposes on imports of U.S. cars.

Republican and Democratic lawmakers have panned the administration’s national security investigation of auto imports and any potential auto tariffs that could result from it. At a hearing on Wednesday, Senate Finance Chairman Orrin Hatch (R-Utah) described how a 25 percent tariff on an average-priced imported car could erase 10 percent of a median U.S. household’s $59,000 income.

“That’s why I call tariffs a tax on American families,” Hatch said.

Even Sen. Sherrod Brown (D-Ohio), a stalwart defender of Trump’s tariffs on steel and aluminum — which were imposed for national security reasons — questioned how the administration could apply the same logic to auto tariffs. It “makes it hard to defend national security issues on steel if you apply it much beyond that or if you aim it at Canada,” the Ohio Democrat said after same hearing.

The German auto industry supports talks to reduce auto tariffs between the U.S. and the EU, but only as part of a larger trade negotiation.

“The basis for future agreements between the EU and the U.S. must be the rules of the WTO,” the German auto industry group VDA said in a statement Thursday. “There is no proposal for unilateral concessions or the mutual dismantling of exclusive car duties.”

The statement followed a Wall Street Journal report on Wednesday that U.S. Ambassador to Germany Richard Grenell was bringing a proposal to Washington from German automakers to eliminate auto tariffs across the Atlantic. While the German auto industry can advocate for such a move, it can’t technically offer any sort of a deal on tariffs, since the EU negotiates trade agreements as a bloc.

German Chancellor Angela Merkel, when asked last week how she would react if Trump went through with his plan to impose tariffs on EU car exports, said: “First we are trying to see if we can prevent that from happening — but I can’t predict this, so I don’t want to raise any false hopes. And then, the European Union will hopefully act as united again, as it has done now.”

Merkel, who made the remarks on German public television, cited the EU’s retaliatory tariffs on bourbon and Harley-Davidsons, and said: “We don’t let ourselves be taken advantage of again and again.”

Another one of Trump’s Friday morning tweets was directed at Mexico, and he took on a much friendlier tone than usual when discussing the NAFTA partner’s trade and immigration policies. Trump praised Mexico’s approach to combating illegal immigration, and even said that Mexico’s reliance on the U.S. as an export market was acceptable.

“80 [percent] of Mexico’s Exports come to the United States. They totally rely on us, which is fine with me. They do have, though, very strong Immigration Laws,” Trump posted on Twitter, a day after a vote on a compromise immigration measure was postponed in the House. “The U.S. has pathetically weak and ineffective Immigration Laws that the Democrats refuse to help us fix. Will speak to Mexico!”

Jakob Hanke and Sabrina Rodriguez contributed to this report.