The trade war between the United States and China spiraled out of control last week, with threats from the United States of yet more tariffs — this time on shoes, clothes and cellphones — and a hasty determination to name China a currency manipulator at least five years after it stopped doing so.

China responded by halting purchases of American farm products and accusing the United States of deliberately destroying the international order. The stock market plummeted, as fear grew that there was no limit on Trump’s willingness to recklessly endanger the American economy. But there is a limit — Congress. And unless Congress steps up, Mr. Trump will continue to flout the rules.

The Constitution gives Congress, not the president, the power to impose tariffs, or taxes on imports. Admittedly, Congress has handed over a good deal of that tariff-levying authority to the president over the years, but there are still important limits in place that Mr. Trump seems determined to ignore.

When imposing tariffs, for example, the usual practice is to give an economic justification. In April 2018, Mr. Trump’s trade representative, Robert Lighthizer, did just that when he announced an intention to impose tariffs on $50 billion in goods from China, declaring that amount to be equal to an expert assessment of the economic harm from China’s policies and practices. The rationale was set out in a well-documented report, as required by Section 301 of the Trade Act of 1974. But just two days later, without presenting further evidence, Mr. Trump threatened to subject $100 billion more in Chinese imports to a 25 percent tariff.