At a rally in Walker, Mich., Monday night, presidential candidate Donald Trump vowed to slap a punitive tariff as high as 40% on Ford Motor f for its manufacturing activities in Mexico.

Ford announced a $2.5 billion expansion of operations in Mexico in April—a decision that soon drew fire from Trump. The car company has become a constant campaign issue for Trump, beginning with his announcement speech in June.

At the time, he told supporters that he’d call up Ford CEO Mark Fields to say: “‘Let me give you the bad news. Every car, and every truck and every part manufactured in this plant that comes across the border, we’re going to charge you a 35% tax. Okay? And that tax is going to be paid simultaneously with the transaction, and that’s it.’”

The rally in Michigan Monday night upped the ante slightly, with Trump promising to raise that tariff by another 5% if Fields doesn’t cooperate, according to the Detroit News. “I’m a free trader, but we can’t be stupid traders,” he said at the rally.

Trump’s proposed tariff would almost certainly violate the North American Free Trade Agreement (NAFTA)—a pact that Trump has vowed to break, anyway.

Ford responded to the attacks in October, after Trump falsely claimed that the company was backtracking on its expansion plans. In an earnings call with investors, Fields said, “Unfortunately, we suspect the facts are getting lost in the politics,” according to the Associated Press.

Fortune has reached out to Ford for comment.