Technically speaking, the secretary is right: Net exports added to growth in the first quarter, according to the preliminary number the Commerce Department released last month. But that is not an indication that tariffs are helping the economy over all.

The fact that Mr. Trump and his team appear to believe otherwise could be a smart negotiating tactic with China — it could give credence to the idea that they are prepared to escalate the trade war further if their demands are not met.

But claiming to have a stronger economic hand could also undermine Mr. Trump’s position and set up the economy — and financial markets — for an unpleasant surprise if his bluff is called.

Here’s why.

Trade hurt economic growth in 2018

The formula for calculating the size of the American economy, the gross domestic product, is deceptively simple. It combines a number of economic metrics, including consumer spending, business investment and government spending. And it also factors in the difference between the value of what the United States exports and what it imports.

For more than 40 years, that difference has been negative as America bought more foreign goods and services than it sold. Imports exceeded exports, which means, technically speaking, that America’s gross domestic product was lower because of trade.