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Charles and Kathleen Moore have done well, but they certainly aren’t billionaires. Yet the couple’s constitutional challenge stands to slam shut the door on a federal wealth tax like the one Sen. Elizabeth Warren wants to enact.

The story is complicated, though less so than the tax code. In the 1990s Mr. Moore, a software engineer, worked at Microsoft on its Office applications and grew close to a fellow programmer, Ravi Agrawal. Mr. Agrawal dreamed of returning to his native India to do something for the small-scale farmers he knew growing up in the state of Chhattisgarh.

On a series of trips to India in the early 2000s, he saw an opportunity. Unlike the massive agricultural operations that feed the U.S., capital-poor farmers working a few acres each serve much of India. What struck Mr. Agrawal is that their tools were plainly inadequate, far less reliable and effective than what any American could buy for a few dollars at Home Depot. His idea was to close the gap by providing India’s poorest farmers with tools that would improve their livelihoods and lives, even in the face of the labor shortages in many rural areas as workers migrated to the cities.

Mr. Agrawal needed capital to get the business off the ground. He approached friends to invest in his new company, KisanKraft, and the Moores put up $40,000. It was a lot of money for them, but they believed in Mr. Agrawal and the mission. They knew they were unlikely to earn much of a financial return on their investment, because the plan was to reinvest any profits in the business and serve more of India’s rural poor.

That was the real return, and it proved massive. Mr. Agrawal had put his finger on an unmet need, and by 2017 KisanKraft had expanded to reach the entire country, with hundreds of employees, thousands of dealers and millions of customers. The Moores have never received a dime from their investment, yet it paid off beyond their greatest hope.

Then the tax bill came. As part of the Tax Cuts and Jobs Act of 2017, Congress reworked the way multinational corporations are taxed, limiting the amount that they had to pay on foreign income. Offsetting part of the cost was a new, one-time tax on earnings that certain foreign corporations had accumulated over the preceding 30 years but not distributed to their shareholders through dividends. The law deemed those earnings as 2017 income to the shareholders and taxed them on it. The Moores’ bill amounted to $15,000. They paid and are now suing for a refund, on grounds that the new tax is unconstitutional.

The Constitution grants Congress the “power to lay and collect taxes,” but with limits. Article I requires that any “direct tax”—one that falls directly on the payer rather than being passed on to someone else, such as the consumer—“be apportioned among the several states” according to population. The idea was that taxation, like representation, should be fairly apportioned so that no state or region could be singled out for disadvantage. Alexander Hamilton explained in Federalist No. 36 that tax apportionment was a key component of federalism, given that direct taxes could disrupt local economies in ways federal lawmakers couldn’t even imagine. By contrast, men of commerce would understand the effects of indirect taxes like tariffs or sales taxes, which the Constitution therefore didn’t subject to apportionment, only uniformity.

The Supreme Court held the first income tax unconstitutional as an unapportioned direct tax in 1895, and Congress eventually responded by proposing the 16th Amendment, ratified in 1913. It authorizes Congress to tax “incomes, from whatever source derived, without apportionment.”

So far as tax law goes, the Moores’ argument is straightforward. The new tax is a direct tax, and it isn’t on income—after all, they haven’t received any from KisanKraft. Instead, they’re being taxed on their property, the KisanKraft shares. The tax is therefore constitutionally invalid because it isn’t apportioned.

The government insists that the Moores are being taxed on income, because KisanKraft could theoretically distribute its accumulated earnings in the future. The courts, however, have consistently defined “income” to require, as the Supreme Court put it in Commissioner v. Glenshaw Glass (1955), “undeniable accessions to wealth, clearly realized, and over which the taxpayers have complete dominion.” As the Moores observe, they haven’t realized a dime in income. The government argues that the courts should abandon the realization requirement, giving the federal government carte blanche to tax “deemed” income without apportionment.

The stakes of the Moores’ case go well beyond their own tax liability. If they prevail, that would confirm that the Supreme Court’s precedents generally requiring apportionment and limiting the exception for taxes on “income” to its common understanding remain good law, clearly barring any kind of federal property tax, including a wealth tax—unless Congress apportions it, which there is no obvious way to do.

What makes the case an especially attractive vehicle to resolve this issue is the simplicity of their situation, a rarity in tax cases. There’s also the timing: If the courts confirm the 16th Amendment’s limited reach now, that would relieve them from having to do so in a politically explosive case directly challenging a wealth tax. The courts would do well to remind Congress at this opportune time that its taxing power is not without limits.

Mr. Rivkin served at the Justice Department and the White House Counsel’s Office in the Reagan and George H.W. Bush administrations. Mr. Grossman is an adjunct scholar at the Cato Institute. Both practice appellate and constitutional law in Washington. They represent the Moores in their refund action.

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