This leaves open the possibility that Mr. Trump could do indirectly what he can’t do directly, by conditioning federal assistance to the states on their compliance with his order to reopen their economies. Unfortunately for him, Chief Justice John Roberts’s decision in NFIB v. Sebelius (2012), which invalidated a portion of the Affordable Care Act, slams the door on this strategy. Conditioning existing federal Medicaid funding on states’ expansion of Medicaid eligibility, he argued, represented a qualitative change in the relationship between the federal government and the states, crossing the line between an incentive and outright coercion. To borrow from “The Godfather,” the federal government was making the states an offer they couldn’t refuse—a clear violation of the Constitution.

Conditioning federal assistance on states’ compliance with a presidential order would be even more coercive today, because the Covid-19 emergency makes states especially dependent on help from the federal government. State attorneys general would rush to federal court to block this strategy, as they would a direct presidential order, and no doubt a judge would issue an emergency injunction.

Beyond these legal considerations lie political realities. If Mr. Trump were to issue a national back-to-work order that purported to override the 42 governors who have issued some version of stay-at-home orders, a substantial number of his grassroots supporters would follow his lead. Some people would congregate in public spaces, while others would reopen their businesses, whatever the effect on Covid-19 infection rates. This would upend plans for a phased reopening that many states are developing and risk creating a chaotic situation with no clear lines of authority.