“SECRETARY CLINTON supported virtually every one of the disastrous trade agreements written by corporate America,” Sen. Bernie Sanders (I-Vt.) railed on Sunday night. As the presidential campaign moves to the Rust Belt, the socialist contender is doubling down on protectionism. His stance is built on bogus numbers that defy the overwhelming consensus among economists, and his solutions would do much more harm than good. But politically, it appears to be working.

As Mr. Sanders won the Michigan Democratic primary on Tuesday, exit polls indicated that his anti-trade agenda helped him. Fifty-seven percent of those who voted agreed that trade “takes away U.S. jobs,” and 56 percent of them picked Mr. Sanders. Those figures augur poorly for Hillary Clinton’s prospects in the Midwest — and for the political durability of the country’s commitment to free trade.

That’s unfortunate, as Mr. Sanders’s populist rhetoric doesn’t stand up to scrutiny. His insistence, for example, that the North American Free Trade Agreement led to 800,000 job losses ignores analyses from unbiased sources such as the Congressional Research Service. “In reality, NAFTA did not cause the huge job losses feared by the critics,” a 2015 CRS analysis found.

Blaming freer trade for the loss of manufacturing jobs fails to tell the much bigger story of economic transformation that has swept the world over the past several decades. Technological change, automation, productivity improvements and other factors have eliminated old-school manufacturing jobs all over the world. Mr. Sanders cannot bring back the U.S. economy of the 1960s, and it would be harmful to try.

Mr. Sanders’s story also neglects to mention the broad benefits that free trade brings. It pulls foreign trading partners out of poverty. It helps U.S. exporters, who account for an increasingly large share of American output. It enriches U.S. consumers, who get cheaper goods and greater selection. Economists resoundingly agree that these sorts of diffuse benefits outweigh the costs over time. Moreover, there are non-economic benefits, as well. NAFTA helped turn Mexico from an antagonist into a regional partner. The much-maligned Trans-Pacific Partnership (TPP) would knit the United States into the Asia-Pacific region for decades to come.

The downside is that freeing up trade clearly exacts some costs in particular sectors, and those affected can be politically mobilized. The solution, though, is not to provoke international enmity and trade conflict by adopting a hostile attitude toward would-be trading partners. The smart policy is embracing openness and pocketing the overall gains in wealth and prosperity, while maintaining a safety net to help those who lose out.

Unfortunately, Ms. Clinton has failed to make these arguments, though they were a gospel of Bill Clinton’s presidency. Instead, she has surrendered the issue by failing to stand up for the TPP. Mr. Sanders already has the protectionists’ vote; Ms. Clinton would be wise to offer a positive alternative.