Like many tweeted proclamations, Trump didn’t provide a lot of details about his plans, leaving us to speculate. Federal quarantine rules are old and have rarely been used, leaving much about them unclear.

Most quarantine power—that is, the power to restrict the movement of people who might have been exposed to a contagious illness, even if they’re not confirmed to be infected—belongs to states. New York, which is experiencing one of the largest Covid-19 outbreaks in the world, clearly could order a mandatory quarantine of individuals exposed to the coronavirus, for instance.

But the federal government has some quarantine power too—and the limitation of federal rules just to “interstate” spread of disease may be broader than it seems.

These rules have rarely been used, so their boundaries have been little tested in court, possibly resulting in “constitutional conflict that could be too fast-moving for courts to intervene,” wrote Polly Price, an Emory University law and public health professor, in the Atlantic last month.

Public health officials prefer voluntary measures, so the federal rules are as a practical matter “relevant only to air travel” and used mostly for tuberculosis patients, Price noted in 2018 in the Emory Law Journal. But she added that “Congress (and the Supreme Court) envisioned a far greater role for federal intervention in matters of state quarantine than we assume today.”

According to US law, the director of the Centers for Disease Control and Prevention can order quarantined a person who is “reasonably believed” to be infected. But that applies only if that person is crossing state lines—or might infect someone else who would cross into another state, potentially spreading the disease further.

That last clause is likely “pretty easy to satisfy,” wrote Robert Chesney in Lawfare last month.

And “if the federal government determines local efforts are inadequate,” it may be able to step in, according to a 2014 Congressional Research Service report (pdf).

However, the law limits quarantine power to individuals—possibly posing a legal problem for a Trump administration attempting to quarantine whole regions, says lawyer Bradley Moss on Twitter. A wholesale ban on entering or leaving an affected area is often called a “cordon sanitaire”—but that’s not a legal term in US federal law.

The CDC director is Robert Redfield. His boss is the secretary of Health and Human Services, Alex Azar, who in turn works for Trump. Given his apparent adherence to expansive “unitary executive” theories of his power (like when he dubs himself the chief law enforcement officer of the US), Trump may believe he has the power to order Azar to order Redfield to require a quarantine order.

(The federal government also has the right to quarantine people who are entering the country from abroad. It already used that power with several individuals arriving from parts of China affected by the coronavirus.)

The same laws that allow the federal government to order a quarantine, however, also require that it provide “adequate food and water, appropriate accommodation, appropriate medical treatment, and means of necessary communication” to the quarantined.

The federal rules further mandate that a quarantine be the least restrictive possible measure to enforce the government’s goals. And quarantine orders can be challenged in court.

This month an opinion piece in the New England Journal of Medicine called for “newer, more creative legal tools” and “public health laws that emphasize support rather than restriction,” despite the “allure of travel bans and mandatory quarantine.”

CBS News White House journalist Fin Gomez reports that Trump may have been inspired by a conversation with Florida governor Ron DeSantis, who this week required any New Yorkers (along with Louisianans) arriving in his state to self-quarantine for 14 days. That order—since it’s not limited to New Yorkers who are specifically thought to have been exposed to coronavirus—may be subject to a legal challenge.

Even though quarantine powers mostly belong to the states—and the governors of New York, New Jersey, and Connecticut are all Democrats who don’t exactly get along with Trump—it’s still possible to imagine that the governors would go along with the president’s proclamation. Even if they see Trump as responding far too late to the crisis, they could avoid turning the pandemic into a one-on-one political battlefield, the kind of fight with which, unlike quelling a pandemic, the president seems to feel comfortable.

It’s a problem that Price, the public health expert, seems to have anticipated in 2018:

Any intervention by the federal government over the objections of a state or territory can be highly politically charged. Environmental, scientific, and medical recommendations can be hijacked for political purposes at any level of government. Perhaps the most pressing problem is a crisis of trust in governmental institutions.

This story has been updated with news of Trump backing off of the quarantine threat.

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