An escalating trade war and steep tariffs on steel and aluminum is putting pressure on earnings for automakers, prompting General Motors to slash its outlook while also weighing down shares of Ford Motor Co. and auto parts companies.

GM cited "recent and significant increases in commodity costs" for why it now expects 2018 profits of $5.14 a share, down from its previous forecast of $6 and below analysts' expectation of $6.42.

Last month, German automaker Daimler AG lowered its 2018 earnings outlook, due in part to increased import tariffs for U.S. vehicles in China. The company said it now sees fewer SUV sales and higher costs at its Mercedes-Benz Cars division than previously expected as a result of the tariffs, and "this effect cannot be fully compensated by the reallocation of vehicles to other markets." Daimler produces vehicles in the U.S.

Ford will report second-quarter earnings after the market's close. Shares were down most of the day, to close at $10.51 per share. GM's shares fell 1.8 percent to close at $37.66.

President Donald Trump in late May imposed steep tariffs on steel and aluminum coming out of Canada, Mexico and the European Union. The 25 percent tariff on steel and 10 percent tariff on aluminum, which took effect in June, have driven up costs sharply as domestic producers raise prices.

"Our biggest exposure, our biggest unmitigated exposure is really steel and aluminum when you look at all of the commodities," said GM CEO Mary Barra on Wednesday. "And frankly, the biggest driver of that is steel."

Trump's latest plan is to consider slapping tariffs on imported autos and auto parts — a move he says would aid American workers but that could inflate car prices, make U.S. manufacturers less competitive and draw retaliation from other nations.

The Center for Automotive Research last week estimated that the potential import taxes of 20 percent to 25 percent on autos and auto parts would raise the price of an average car by $4,400. The duties would eliminate more than 714,000 U.S. jobs.

The president has ordered the U.S. Commerce Department to investigate whether auto imports pose a threat to U.S. national security that would justify tariffs or other trade restrictions. Earlier this year, he used national security as a justification for taxing imported steel and aluminum.

Last week, critics from the auto industry lined up at an all-day hearing with the Commerce Department to urge the administration to reject auto tariffs. They argued that the taxes would raise car prices, squeeze automakers by increasing the cost of imported components and invite retaliation from U.S. trading partners — and allies — like the European Union and Canada.

Auto tariffs would escalate global trade tension dramatically: The U.S. last year imported $192 billion in vehicles and $143 billion in auto parts — figures that dwarf last year's $29 billion in steel and $23 billion in aluminum imports. Not to mention the $34 billion in Chinese goods the administration has so far hit with tariffs in yet another dispute over the predatory practices China deploys in a push to challenge U.S. high-tech dominance.

Leaders in the auto industry have been raising concerns over the tariff impact for months.

"Imposing tariffs will increase costs for consumers, lessen consumer choice, lower consumer demand, reduce car and light truck production and sales, lower investment levels, and lead to job losses in the U.S. auto sector," said Matt Blunt, president of the American Automotive Policy Council, in June.

Other industries are also feeling the pain. Harley-Davidson says it expects new tariffs to increase the company's annual costs by as much as $100 million as long as the trade dispute between the U.S. and other countries goes on.

Executives with the Milwaukee company spoke with investors Tuesday for the first time since announcing last month that production of motorcycles sold in Europe would move overseas in order to avoid retaliatory tariffs the EU is imposing on American exports.

Harley-Davidson said it's working with the Trump administration and other governments to try to get the tariffs removed.

In the short term, the cumulative impact will increase Harley-Davidson's costs as much as $55 million this year, the company said. Costs from raw materials subject to tariffs, like steel and aluminum, account for $15 million to $20 million, and the EU tariffs add another $30 million to $35 million, according to Harley-Davidson.

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Damian J. Troise in New York and Chris Rugaber in Washington contributed to this report.