In the past, the one constituency President Trump has sometimes listened to has been our stock market. Well, it has spoken, falling 10.5 percent in one of the largest two-day stock market swoons in decades.

In the 50 years I have been immersed in markets and economic policy, I have never before witnessed a signature economic policy initiative that was met with such unalloyed criticism. What’s worse, the damage was entirely self-inflicted.

Why such a reaction? One reason the S&P 500 fell was that the tariffs Mr. Trump rolled out were so much greater than investors anticipated. (Give the White House an F for failing to prepare the market for what to expect.) Then on Friday, China announced its own 34 percent tariff on our goods, making it clear that our trading partners were not going to simply give in to Mr. Trump’s demands, as he had suggested they would.

As Mr. Trump was doubling down, asserting that “my policies will never change,” the Federal Reserve chairman, Jerome Powell, was delivering his own bombshell: Given the higher-than-predicted tariffs, higher inflation and slower growth were likely to ensue, he said. That’s drastically different from just a couple of weeks ago, when Mr. Powell called the potential impact of new tariffs on prices “transitory.”

The business community, which by my count heavily supported Mr. Trump in the election five months ago, seems stunned. Few have spoken publicly, but the Business Roundtable, the premier corporate trade association, on Wednesday warned that universal tariffs run “the risk of causing major harm to American manufacturers, workers, families and exporters.”

Privately, several chief executives told me that they recognized that imposing the tariffs, as well as Mr. Trump’s intractable support of them, was a potentially cataclysmic mistake. “Few of us ever imagined he would go this far,” one told me. “He could well bring down the economy and himself.”

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