Dr. Seru is a finance professor at Stanford Graduate School of Business.

Donald Trump casts himself as a champion of American business. In many ways, he is. He has slashed corporate taxes. He is rolling back regulations and embracing industries like artificial intelligence. Yet the payoff is far smaller than many business leaders expected, and our economy is losing momentum.

The reason is simple. Mr. Trump’s constant policy swings are offsetting whatever benefits his business-friendly instincts might bring. There is some irony that a president who was elected in part because of his perceived business acumen is instead intensifying one of the most corrosive forces in the economy: the creeping of political........

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