Is President Donald Trump bent on political self-harm? It often seems that way. His overall job approval rating still hovers within a point or two of the 50% popular vote he received last November. But he is losing support on the economy and inflation, the No. 1 issue last year, while his overwhelming success in reducing illegal immigration has reduced the salience of what was the No. 2 issue.

Tariffs make things cost more. If you slap a 25% tariff on imported automobiles, they will cost more than they used to, even if sellers absorb some of their increased costs. Sellers of American-made cars are likely to cost more too. That’s because sellers can raise their prices some distance even while gaining market share.

Voters understand this. And they don’t enjoy price increases.

When the Biden administration’s spending programs led to sharp price increases, voters were assured they were just “transitory.” That turned out to be wrong. Prices never went back down.

More importantly, America had not experienced serious inflation since the 1970s, meaning no voter under 65 had any memory of facing sharply increasing prices.

Now the Trump administration is explaining that increased prices caused by tariffs will be temporary — another four-syllable word that starts with T. Voters seem skeptical, while investors seem spooked by the president’s rapidly changing threats. Trump’s job approval on the economy has tumbled.

America has not experienced sharply increasing tariff rates since the passage of the Smoot-Hawley Tariff Act in 1930. No voter has any memory of that.

Did any voters expect Trump to set 25% tariffs on imports from Canada? Did any significant number want that? Did any Trump voter want to see Canada become the 51st state?

Trump’s referring to former Prime Minister Justin Trudeau as “Governor” may strike some as funny, but it amounted to political self-harm. Trudeau resigned, and now Pierre Poilievre’s Conservative Party, once far ahead in polls, is essentially tied with the Liberals in the snap election called for April 28. Moreover, both parties are now pledging to retaliate with high tariffs on imports from the United States.

The fact is that the U.S. and Canada are pretty much one country economically and have been for a long time.

Over time, we’ve become more tightly linked. The germ of the 1993 North American Free Trade Agreement was the Canada-U.S. Automotive Products Agreement of 1965 — 60 years ago! — sought by the Detroit-based Big Three (as they were then) automakers and Walter Reuther’s United Auto Workers, then the leading creative force on the political left.

Before World War II, the Big Three had built a separate Canadian line of cars as they had in Europe. It made good sense to meld the supply chains and assembly plants over the Michigan-Ontario border (as it had over the Michigan-Ohio border) and to eliminate cross-border tariffs.

As a result, the Detroit Three’s supply chains centered in Michigan extend east into Ontario just as they do south to Ohio and Indiana. This is of huge importance to Canada. About one-quarter of its foreign trade passes over the Detroit River on the Ambassador Bridge, and, given the bridge’s age (it opened in 1929) and high tolls, the Canadian government is paying for the construction of the new Gordie Howe International Bridge.

Suddenly imposing tariffs on autos and parts crossing the U.S.-Canada border wreaks destructive havoc on the Detroit Three and its major suppliers on both sides. It threatens to put them at a commercial disadvantage to the foreign-based automakers whose assembly plants and supply chains extend south from Ohio and Indiana through Kentucky and Tennessee into South Carolina, Alabama and Mississippi.

The Trump tariffs also threaten to increase costs for purchasers of Detroit Three autos, who are concentrated geographically between the Appalachians and the Coast Range and politically among Trump and Republican voters. In my light, this policy is a textbook case of political self-harm.