On the campaign trail, Donald J. Trump has promised to do quite a few things that are beyond the powers of an American president, like billing Mexico for a border wall. But when it comes to foreign trade, his powers as president would come closer to his expansive ambitions.

As president, Mr. Trump could seek to penalize other nations for undercutting American manufacturers or stealing American ideas. He could also pursue congressional legislation to impose a 45 percent tariff, or tax, on imported Chinese goods, as he has proposed.

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“It would be a flagrant violation,” said Alan O. Sykes, a professor of law at Stanford and an expert on international economic relations. “There is no prior violation of W.T.O. law that would be even close.”