"In a government system with checks and balances, the president should not have the power to unilaterally levy or alter tariffs."

In a letter to the Trump administration, the lawyer representing the health insurer said Idaho's plan was "entirely legal."

This is likely to have serious negative consequences for the American economy and put millions of jobs at risk.

Cohn, who opposed such trade measures in favor of a free-trade approach, declined to concede.