Washington — A divided Supreme Court on Wednesday appeared skeptical of President Trump's authority to unilaterally impose tariffs on nearly every country under a federal emergency powers law, as the justices tested a centerpiece of his economic agenda and the limits of presidential powers.

The dispute over Mr. Trump's sweeping tariffs marks the first in which the Supreme Court weighed the legal merits of one of the president's signature policies implemented in his second term. Arguments spanned nearly three hours, with the justices asking probing questions of lawyers for the Trump administration and a group of small businesses challenging the legality of the president's tariffs.

But several of the justices appeared uncomfortable with the breadth of power Mr. Trump is asserting with

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