The order by Governor Raimondo of Rhode Island to have state police pull over all cars entering the Ocean State with New York license plates opens a new front in the war on the coronavirus. It’s not our purpose here to quarrel with the governor. She leads the second most densely populated state in the nation. Florida, Maryland, Texas, and South Carolina are reportedly also targeting New Yorkers. These policies, though, skate close to a constitutional red line.

That’s because traveling freely among the several states is enshrined in constitutional case law as a fundamental right. The courts have differed over the years on which clause of the Constitution the right to travel among the states is seated — the equal protection clause, say, or the privileges and immunities clause. If the right is lost or trifled with, though, the very fabric of America would be frayed or torn.

We understand that these are extraordinary times. A deadly disease is on the march, and New York City is, at least for the moment, an — even the — epicenter. Thousands of New Yorkers are heeding the advice to maintain social distancing by retreating to the country, where populations are far less dense. No doubt some more rural states and towns have grown alarmed. The constitutional record is clear, though, the right to travel between states is fundamental.

This was most famously marked by a case brought by a Pennsylvanian, Edward Corfield, whose fishing boat, the Hiram, was seized by New Jersey, which forbad out of state persons from taking its oysters. The case, Corfield vs. Coryell, was, in 1823, heard by a Supreme Court Justice, Bushrod Washington. He concluded the oysters were state property and New Jersey could favor its citizens. The right of travel, though, he marked emphatically.

The justice provided a shortlist of “the privileges and immunities of the several states.” He confined it to liberties that “are, in their nature, fundamental” and belong to citizens “of right.” His shortlist featured “the right of a citizen of one state to pass through, or to reside in any other state, for purposes of trade, agriculture, professional pursuits, or otherwise.” Cornell Law calls it “a right venerable for its longevity.”

Over the years, our constitutional judges have noodled over this a good bit (that’s what they do for a living, thank God). None, though, has managed to reverse, or significantly narrow, Justice Washington’s declaration in the Corfield case. So when Rhode Island starts telling its cops to keep an eye out for New Yorkers, it warrants at least a pause to reflect on the glue that holds our republic together.

It happens that the Constitution does acknowledge the problem of inspection at state borders. It does so in Article One, Section 10, which lists things states may never do. One of them is “lay any Imposts or Duties on Imports or Exports,” the parchment says, “except what may be absolutely necessary for executing its inspection Laws.” Whether the courts might use such a clause to empower Rhode Island is hard to guess.

We’re inclined, watching the broadcast by Ms. Raimondo, to trust the governor’s judgment. Her measures are targeted to those who enter Rhode Island from New York with the intent to stay there. Then they will be required to self-isolate for 14 days. The broader point bears watching. If the right to travel among the states gets frayed, the next test will be intrastate travel, which is not vouchsafed by the Constitution.

That is, it hasn’t been declared a fundamental right. Already in New York, some towns — in the Hamptons, say — are trying to forbid entry by New York City residents with vacation or rental homes. If that kind of thing gets out of hand, America is going to start to lose the sense that we’re all in this together. The spirit of e pluribus unum will be lost. And, in President Trump’s phrase, the cure really could end up worse than the disease.