The head of a U.S. manufacturers association called President Donald Trump’s plan to impose new tariffs on Mexico an explosive “Molotov cocktail” policy.

“Intertwining difficult trade, tariff and immigration issues creates a Molotov cocktail” policy that would have “devastating consequences on manufacturers in America,” warned Jay Timmons, president and chief executive officer of the National Association of Manufacturers.

Trump threatened via tweet on Thursday to impose the tariffs not for business reasons but to force Mexico to stop people from entering the U.S. illegally. Tariffs would start at 5% on June 10 and gradually ramp up to 25% in October, he tweeted.

The threat of a “second front” in Trump’s trade war on top of ramped-up tensions with China sent the stock market spiraling downward.

Trump raised the Mexico tariff issue even as Republicans were pressing to pass the U.S.-Mexico-Canada Agreement, which the administration negotiated to make revisions to 1994′s North American Free Trade Agreement. The revised accord may be in jeopardy now.

The U.S. Chamber of Commerce is considering a legal challenge to the threatened new tariffs.

Despite mounting concerns, Trump doesn’t seem to be changing his mind. He gushed in a tweet Saturday that “tariff is a beautiful word indeed!”