Republican donor Charles Koch slammed President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE's announced tariffs on steel and aluminum and encouraged corporate leaders to reject the plan in an op-ed on Wednesday.

"One might assume that, as the head of Koch Industries — a large company involved in many industries, including steel — I would applaud such import tariffs because they would be to our immediate and financial benefit. But corporate leaders must reject this type of short-term thinking, and we have," Koch wrote in The Washington Post.

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Koch went on to describe the "harmful effect" he said the tariffs would have, saying consumers will be hurt the most by the restrictions.

Koch is a free market libertarian who did not contribute to Trump's campaign despite his history as a major Republican donor. He is the latest businessman to speak out against Trump's plan to slap a 25 percent tariff on imported steel and a 10 percent tariff on imported aluminum.

Republicans have not lined up behind Trump's announced tariffs either.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE (R-Ky.) said on Tuesday he has "genuine concern" the tariffs could lead to a trade war.

"I think the best way to characterize where most Republican senators are right now, including myself, is genuine concern that this not escalate into something much broader," McConnell said.

The European Union has said it will consider slapping its own tariffs on major U.S. imports such as bourbon and Harley-Davidson motorcycles, which could have major impacts on McConnell's and Ryan's home states.

However, Trump has maintained that tariffs will help the U.S., which he said has been on the losing side of trade.