U.S. President Donald Trump's tariff regime faces its biggest legal test yet this week.

The U.S. Supreme Court will hear arguments Wednesday on Trump's unprecedented use of an emergency powers law to slap broad-based tariffs on imports from Canada and other major trading partners.

On social media Trump has called it “THE MOST IMPORTANT CASE EVER" and repeatedly claimed that the Ontario government's anti-tariff ad campaign featuring an address by the late Ronald Reagan was an attempt to interfere with the case .

Many billions of dollars are at stake. If the administration loses in the Supreme Court, it may have to return tariffs paid by importers since the spring and will forgo a source of revenue that Trump has claimed is making America rich again.

Elizabeth Wydra, president of th

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