President Trump has invoked Section 338 of the Tariff Act of 1930 to impose a 50 percent tax on certain Canadian imports, reprising a strategy of using decades-old, never-before-used trade powers that has repeatedly drawn federal court challenges.

No president appears to have issued tariffs under Section 338 before, raising novel legal questions about whether the statute was effectively rendered obsolete by later congressional action. Trade lawyers note the law has never been tested in court.

The Trump administration has already suffered significant legal defeats on tariffs, including the Supreme Court striking down his reciprocal tariffs in February, requiring $160 billion in repaid revenue. A separate tariff imposed under Section 122 was declared illegal by a lower court this spring; the administration has appealed.

Patrick Childress, a partner at Holland & Knight, said it is difficult to assess how vulnerable a Section 338 challenge would be given the absence of any judicial interpretation. Canada has unveiled retaliatory tariffs on U.S. steel, dairy, and clothing starting after Labor Day, promising a "dollar for dollar" response.