Shares of the Big Three automakers slumped on Monday in premarket trade as the impact of 25% tariffs on Canada and Mexico hit the companies hard.

President Donald Trump signed an order to put tariffs on U.S. neighbors Canada and Mexico, as well as China, starting Tuesday.

Trump has long proposed raising tariffs—or taxes on imported goods that the businesses importing them pay the U.S. government—before the election, and said in November he’ll impose a 25% tariff rate on all imports from Mexico and Canada on the first day of his presidency and will impose an additional 10% tariff rate on all imports from China on top of other tariffs.