President Trump escalated his fight with Canada on Tuesday, saying that he would double tariffs on steel and aluminum imports and threatening to inflict even more pain on one of America’s closest traditional allies as he pressed Canada to become part of the United States.

His comments sent jittery markets tumbling, with the S&P 500 down about 1 percent in early morning trading.

In a post on his social media platform, Mr. Trump wrote that Canadian steel and aluminum would face a 50 percent tariff, double what he plans to charge on metals from other countries beginning Wednesday. He said the levies were in response to an additional charge that Ontario placed on electricity coming into the United States, and he threatened more tariffs if Canada didn’t drop various levies it imposes on U.S. dairy and agricultural products.

“If other egregious, long time Tariffs are not likewise dropped by Canada, I will substantially increase, on April 2nd, the Tariffs on Cars coming into the U.S. which will, essentially, permanently shut down the automobile manufacturing business in Canada,” he threatened.

Mr. Trump went on to say that “the only thing that makes sense” is for Canada to become the 51st U.S. state.

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