Twelve years ago, the Supreme Court held in a 5–4 decision in Maryland v. King that police departments could perform cheek swabs on people they take into custody and use the DNA sample for criminal investigations. The case is memorable not only for its ruling but for the vivid dissent by Justice Antonin Scalia and three of the court’s liberals at the time.

Scalia argued that suspicionless searches for criminal evidence, like the cheek swabs in question, cut against the core of the Fourth Amendment’s principles. “Perhaps the construction of such a genetic panopticon is wise,” he wrote, joined by Justices Ruth Bader Ginsburg, Sonia Sotomayor, and Elena Kagan. “But I doubt that the proud men who wrote the charter of our liberties would have been so eager to open their mouths for royal inspection.”

In a strange turn of events, however, the actual decision in King may turn out to be less legally significant than a then–largely unnoticed administrative ruling that Chief Justice John Roberts handed down when the case reached the court. His relatively minor decision at the time laid the groundwork for the court’s whipsaw tilt in favor of the Trump administration’s policies—and how it understands the concept of “irreparable harm” in its shadow docket cases.

Maryland enacted its DNA Collection Act in 1994 at the dawn of the age of DNA forensic testing. While DNA testing methods had been used by law enforcement before the act’s passage, they did not burst into the public consciousness until 1995, when they played a central role in the O.J. Simpson trial. The Maryland law allowed police to take DNA samples from a suspect when they apprehended him for violent-crime charges and compare them to a state database.

Alonzo King, the titular defendant in the case, was arrested in 2009 on assault charges. Police obtained a DNA sample from him while processing him in the local jail. When they compared it to the state database, they found a match with an unidentified sample taken from a 2003 rape case. Thanks to the DNA match, they charged him with the previously unsolved crime and obtained a conviction from a local jury.

On appeal, King argued that the 1994 law violated the Fourth Amendment because it represented a suspicionless search for criminal evidence. Maryland argued (and the five-justice majority on the Supreme Court agreed) that it was permissible because it was akin to fingerprinting, which had been a standard police booking practice over the prior century. The Maryland Court of Appeals sided with King in 2012, prompting the state to ask the Supreme Court to review the case.

After filing a petition for review, Maryland also asked the high court to stay the Maryland Court of Appeals’ ruling pending the outcome of the appeal. This request was processed not through the court’s regular merits docket but through its emergency docket. (Three years later, legal scholar William Baude would coin the term “shadow docket” to describe its workings.)

For the shadow docket, each of the nine justices is assigned to cover at least one of the federal circuit courts of appeals. There are 13 of them—the First through Eleventh Circuits for the states, the D.C. Circuit for the District of Columbia, and the confusingly named Federal Circuit for the special-jurisdiction federal courts. Roberts, by tradition, always takes the D.C. Circuit, the Federal Circuit, and the Fourth Circuit, which covers the states surrounding the nation’s capital.

When a motion to stay a lower court’s judgment reaches the Supreme Court, it first goes to the “circuit justice” assigned to........

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