“There would be no public health benefit,” said Nuzzo, who studies government responses to health crises. “Over 40 states are reporting cases. … The horse is out of the barn. The disease is already here.”

On Friday, Trump invoked his first significant emergency power since the coronavirus first appeared in January, declaring a national emergency.

In its declaration, the White House cited two laws — the 1988 Stafford Act, a disaster relief law, and the 1976 National Emergencies Act, the law Trump used to divert government funds to build a southern border wall.

For days, though, Trump had balked at exercising the power, even as he faced a chorus of calls from health groups, disaster relief specialists and Democrats to do so. According to people familiar with the situation, Trump had been reluctant to take the step because it could undermine his contention that the coronavirus outbreak is similar to seasonal flu.

But he insisted on Thursday that he was more than willing to invoke any of his emergency authorities.

“We have very strong emergency powers under the Stafford Act. … And if I need to do something, I'll do it,” he said, adding cryptically, “I have the right to do a lot of things that people don't even know about.”

Trump has not previously been bashful about exercising his emergency powers. He was only seven days into his tenure when he issued the hugely controversial travel ban, sharply restricting the flow of people to the U.S. from mostly-Muslim countries. A series of court rulings blocked enforcement of the original ban, prompting the administration to redraft the order several times. The Supreme Court ultimately upheld a substantially pared-back version of the initial order.

The policy — which critics called a “Muslim ban” — was based on the same legal authority Trump has since used to exclude foreign travelers from China and, now, from much of Europe, in response to the current virus.

Many White House statements about the new virus-related travel restrictions go out of their way to explicitly cite the precise legal authority tapped in both instances, giving the president the power to “suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens any restrictions he may deem to be appropriate.”

The mentions seem to echo arguments the Justice Department made in the “Muslim ban” case. Lawyers repeatedly warned judges to be wary about restricting the power because it may be needed in a crisis.

Perhaps Trump’s most aggressive executive actions have involved money, namely his moves to spend billions of dollars on construction of a border wall with Mexico despite Congress’s unwillingness to fund that project — a signature of his 2016 campaign — at anywhere near the levels he has sought.

Trump’s lawyers have cited powers Congress gave the president to reallocate military construction funds and to tap other funds for counter-drug activities.

Lower courts halted more than $6 billion of the funding, questioning Trump’s claims of an emergency and noting Congress’s rather explicit refusal to fund expansion of the wall. But the Supreme Court stepped in last year to allow Trump to proceed with the spending as litigation continues.

Historically, officials at the state and local level have taken the most aggressive measures to address public health emergencies.

In 1899 and 1900, officials in Honolulu and San Francisco cordoned off those cities’ Chinatown districts in a bid to control the bubonic plague, a move now viewed as heavily tainted by racism. Authorities in Hawaii even lit one home on fire in order to try to squelch the disease, but in the process burned down much of the neighborhood.

Nothing similar has happened yet in the U.S. during the coronavirus outbreak, although governors are increasingly flexing their legal muscle.

Maryland Gov. Larry Hogan (R) on Thursday ordered the closure of all public schools in the state for two weeks. On the same day, California Gov. Gavin Newsom (D) issued an executive order directing residents to eschew non-essential gatherings of more than 250 people. Newsom also tapped the state’s power to “commandeer” hotels, if needed, as temporary hospitals or quarantine centers.

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And in New York, Gov. Andrew Cuomo (D) declared a “containment zone” in New Rochelle, N.Y., to combat a virulent cluster of corona cases there. He even called out the national guard to assist with food distribution and other logistics in the city.

Trump weighed in on the move at his photo-op Thursday, noting it isn’t a flat-out ban on movement or work. While he called the action “not very strong,” he seemed to welcome the fact that it would give locals the feeling “they’re being watched.”

Some of the more severe travel limits the federal government can impose are quietly wielded in individual cases.

The CDC maintains a little publicized “do-not-board” list for domestic and international airline travel. Details on the list are thin, but it has included people with tuberculosis and measles. In 2015, CDC formally declared that individuals could be added to the list if they were suspected simply of being exposed to hemorrhagic fevers, like Ebola.

Back in 2007, an Atlanta lawyer drew national attention when he flew to Paris to get married and returned to the U.S. while suffering from drug-resistant tuberculosis. Doctors said they told him not to travel, but he insisted they never told him he could be contagious. He was eventually put under a federal quarantine order, but it turned out his disease was not the type most resistant to treatment.

Typically, however, federal officials have relied on state and local health departments to address most disease outbreaks, even sometimes asking them to detain individuals who refuse to cooperate with doctors’ recommendations.

But Trump has hostile relationships with the Democratic governors of the most affected states — Washington, California and New York. And if he wanted, he could have the CDC step in. Federal regulations give the CDC sweeping powers if a state’s measures “are insufficient” to prevent a disease from spreading into another state. The CDC chief is authorized to do what he or she “deems reasonably necessary” under those circumstances. Whether that includes widespread restrictions on movement is unclear.

Parmet, the Northeastern University law professor, stresses that the Constitution still applies. Aggressive federal government action on such vague authority could face legal challenges, especially if the moves don’t have a scientific basis.

On the other hand, in a crisis judges tend to defer to executive authorities, including health officials.

“The real danger is that these moves are taken just for show,” Parmet said. “At this point, where we are in the curve of this pandemic, there’s no magical legal elixir.”