Tariffs are more than just taxes. They are a tool of geopolitics.

With President Biden maintaining many of those tariffs and Trump promising more if re-elected, the criticism has grown. Two new studies show tariffs are regressive, meaning they fall more heavily on lower-income families who tend to spend more of their income on cheap imported goods.

Still, while who pays tariffs is hotly debated, it may not be what matters most. Duties on China aren’t meant to raise money but to reduce U.S. dependence on a potential adversary. In that sense, they aren’t like other taxes, or even like tariffs of previous eras.

In his definitive work “Clashing Over Commerce: A History of U.S. Trade Policy," Dartmouth College economist Douglas Irwin shows that tariff policy has gone through three distinct phases since the 1700s.

Revenue, restriction, reciprocity…

From independence until the Civil War, Irwin writes, the purpose of tariffs was mainly revenue: They accounted for 90% of federal receipts. From the Civil War until the Great Depression, the purpose was restriction: protecting northern manufacturers, then represented by the newly dominant Republican party, from imports.

A third era began with the passage of the Reciprocal Trade Agreements Act in 1934 empowering the president to negotiate lower duties if other countries did the same. Reciprocity remained the dominant paradigm after World War II as presidents of both parties sought to knock down other countries’ trade barriers through a mixture of carrots (trade deals) and sticks (targeted duties and quotas).

Trump’s initial tariffs on solar panels, washing machines, steel and aluminum mixed restriction and reciprocity, protecting some industries while pressuring Canada, Mexico, Japan and South Korea to revise their trade relations with the U.S.

…now realignment

The tariffs he imposed on China and to which President Biden just added are a different animal altogether. They are partly about restriction and reciprocity—protecting nascent industries and prodding China to change its ways. But the more fundamental goal is realignment: diversifying U.S. trade away from China. Its dominance in numerous manufactured goods and processed minerals, officials fear, gives China too much influence over the U.S. and its allies’ economies and, ultimately, security. That fear has grown with the threat of a new “China shock" of cheap manufactured exports.

The office of Katherine Tai, Biden’s trade ambassador, made this clear in explaining last week why many importers, after requesting extensions to their exclusions from the tariffs, were denied. Many “simply asserted that (alternative) products were unavailable because China remained the lowest cost source," it said. Extending their exclusions would simply delay the shift to “alternative sourcing and continue their dependence on Chinese suppliers and products, which undermines the goal of" changing China’s behavior.

Who pays tariffs depends on myriad factors. While several studies found U.S. importers did pay more because of tariffs, those costs weren’t necessarily passed on to consumers. Still, some researchers found the surge of imports from China following its entry into the World Trade Organization in 2001, while displacing millions of American workers, benefited most consumers through lower prices. Logically, tariffs would hurt those same consumers.

Poor people pay tariffs

Economists Amit Khandelwal of Yale University and Pablo Fajgelbaum of the University of California, Los Angeles illustrate this neatly by studying the increase in the “de minimis" exemption, below which small packages may enter the U.S. duty-free, to $800 from $200 in 2016.

The authors found that 74% of direct shipments received in the poorest ZIP Code were de minimis, compared with 52% for the richest. De minimis is controversial: Many importers use it to circumvent the tariffs on China. E-commerce giants Shein and Temu use it to ship from China to the U.S.

Some in Congress, and Trump advisers, want it scrapped. That would hurt the poor, the authors say: People in lower-income ZIP Codes would lose $45 a year, compared with $35 for middle-income ZIP Codes and $81 for the richest.

At present, tariffs represent 2% of the value of imports. That would skyrocket to almost 17%, the highest since the passage of the Smoot-Hawley Tariff in 1930, if Trump is re-elected and carries out his threat to raise tariffs to 60% or more on China and 10% on the rest of the world, according to Sarah Bianchi and Matthew Aks of Evercore ISI, a brokerage.

That would really bite, especially on lower-income households. Kimberly Clausing and Mary Lovely of the Peterson Institute for International Economics estimate this would shrink the purchasing power of the 20% poorest households by 4.2%, but the top 1% by just 0.9%. This doesn’t capture all of the costs, which include less-efficient producers gaining sales at consumers’ expense and disruptions from reorganizing supply chains.

Tariffs as ‘Pigouvian’ taxes

Whether those downsides kill the case for tariffs depends on what they accomplish. Tariffs on China can be thought of as a “Pigouvian" tax. Named for the British economist, Arthur Pigou, such a tax offsets some collateral social harm—much as a carbon tax helps to reduce global warming. Consumers bear a direct cost from tariffs on China, but the U.S. as a whole gets a less-vulnerable, more-diversified supply base. Ending the de minimis exception eliminates a loophole used to avoid those tariffs.

Nonetheless, the distributional consequences are real. One response might be to rebate some of the revenue from tariffs to those hit hardest, as Canada does with its carbon tax. But as Clausing and Lovely note, Republicans’ plan to extend their 2017 tax cut, large parts of which expire in 2025, would do the opposite, delivering disproportionately big benefits to the top 1% and little to the bottom 20%.

Tariffs are more than taxes. They are also an instrument of geopolitical competition. Nonetheless, like all taxes, they impose costs that need to be weighed against their benefits. It isn’t clear what benefit could justify hitting the entire world with a 10% tariff, especially if the world retaliates.

Write to Greg Ip at greg.ip@wsj.com