The U.S. Constitution gives the power to impose taxes — including tariffs — to Congress. But lawmakers have gradually relinquished power over trade policy to the White House. And Trump has made the most of the power vacuum, raising the average U.S. tariff to more than 18%, highest since 1934, according to the Budget Lab at Yale University.

Most of the tariffs that U.S. President Donald Trump has imposed on countries around the world — including the ones on Canada that he's threating to bump to 35 per cent — face a crucial legal test on Thursday.

A U.S. appeals court on Thursday will review President Donald Trump's power to impose tariffs, after a lower court said he exceeded his authority with sweeping levies on imported goods.