One reason Democrats couldn't keep the White House after the Obama era — shockingly losing to Donald Trump — is that the Obama administration had done so much to retard economic growth for so long.

Sure, the 2008 recession was obviously not Barack Obama’s fault. But eight years later, people were genuinely tired of the glacial pace of recovery. Moreover, so many at the lower end of the income scale, and so many in areas where the Obama EPA had stepped in to make examples of local industry, felt they had been left behind. Obama’s White House worked to convince people that anemic growth is the new normal.

By ushering in intense deregulation and especially newly pro-energy policies, President Trump quickly managed to prove that wrong. He has presided over a few quarters of very strong economic growth. But the last quarter wasn’t one of those strong ones, and the reasons why should really give him pause.

Despite excellent employment numbers, strong corporate profits, and robust consumer spending, economic growth was just revised downward last week for the second quarter of 2019 to 2%. That’s a sharp decline from the 3.1% growth of the first quarter. And everyone in the White House, including its most important resident, should be asking why.

No, this isn’t necessarily a sign that recession is here or that even that it’s coming anytime soon. But the downward revision should serve as one additional reminder that the economy could be in much better shape. And it will be, if Trump ever repents of his anti-trade tendencies, pulls back on the tariffs, and lets business do its thing. His own experience in office thus far should have already taught him that trade wars are no good for anyone involved. Protectionism is threatening his presidency. If he doesn’t figure out a way to get America back quickly into the business of trading freely, it will cost him reelection next fall.

As we’ve noted previously, Trump’s conflict with China might be understandable as a security measure. But to the extent that it is trying to force an end to Chinese theft of technology and intellectual property, it is similar to a war: an undesirable measure of last resort that harms everyone involved. Trade wars have no winners, even under the best of circumstances.

The sluggish second quarter of this year serves as still more evidence that tariffs are taxes on American consumers that retard growth. They raise barriers against United States exporters, and they impose new, unexpected, and unnecessary expenses on U.S. importers. They are not good for the domestic economy, they do not revive dormant U.S. industries, and they are not helping Trump’s reelection prospects on whit.

We noted earlier this week that American farmers are suffering badly from the mutual self-destruction of the China-U.S. trade war. Bad policy has turned many formerly self-sufficient farmers into welfare clients.

But the fact that domestic economic growth has been revised downward for everyone else should come as yet another warning. Trade protectionism is a failed economic theory that is as flawed and as failed as its close relative, socialism. If Trump does not offer Americans something better, it is hard to imagine how he can defeat that noxious and destructive system at the ballot box.