The issue isn’t whether Congress can tax wealth; Congress can tax almost anything. But if a tax is a direct tax, the Constitution requires that it be apportioned among the states on the basis of population, regardless of how the tax base is spread across the country. The apportionment rule was intended to make direct taxation difficult and often impossible, and it does just that. If a wealth tax has to be apportioned, Ms. Warren’s tax couldn’t work as intended. The amount that would have to be collected from two states with equal populations would have to be the same, even if one of the states is wealthy and the other poor. That would be crazy, and, as a result — or so one hopes — Congress wouldn’t enact such a tax.