U.S. autos makers and parts suppliers were among the sectors hit hardest in early Friday trading after President Donald Trump’s threat to impose a tariff of up to 25% on Mexican goods. General Motors Co. and Ford Motor Co. each fell more than 3%.

The latest blow in the trade disputes adds to a month that has trimmed 12% from the 24-member S&P Supercomposite Automobiles and Components Index, shaving about $20 billion in market value in May through Thursday. It was on track to be the worst month for the sector since December’s 14% slump, although Friday’s declines could push the group to its worst month since November 2016.

Citi analyst Itay Michaeli sees tariffs having a bigger impact on GM than on Ford, due to its higher production from Mexico on higher-margin vehicles. A 5% tariff could become a “several-hundred-million-dollar” hit to annual earnings for GM, he said. GM fell 4.9% on Friday, while Ford slipped 4.3%.

Among parts makers, Delphi Technologies plunged 8%, Adient fell 4.2%, Goodyear Tire & Rubber lost 2.85, and Aptiv dropped 2.7%. Michaeli also named American Axle, Lear, Magna International and Visteon among suppliers with the biggest exposure to Mexico.