Who — or what — is Justice Amy Coney Barrett? The question nags at us after yet another surprising opinion from the star of Notre Dame. The latest curveball from Justice Barrett arrived Monday evening in the case concerning the Trump administration’s use of the Alien Enemies Act of 1798 to expel to El Salvador alleged members of the gang called Tren de Aragua. Justice Barrett landed in large part in league with the liberals.

Wonder what will be made of that by the leftists who protested so noisily at Justice Coney Barrett’s  confirmation hearing. Capitol Police arrested and charged 21 people with “crowding, obstructing, or incommoding,” and one with unlawful conduct. Every Democrat in the Senate voted against her confirmation. Senator Blumenthal called it a “raw exercise of power,” coming as it did at the tail end of President Trump’s tumultuous first term.    

We don’t mind saying that to us, it looks as if America is seeing a swing vote take shape in real time. The role was once been filled by the likes of Justices Anthony Kennedy and Sandra Day O’Connor. Justice Kennedy, nominated by President Reagan, became a swing vote in, say, Obergefell in 2015 and Shelby County two years prior. Justice O’Connor, another Reagan nominee, was more reliable but not exactly predictable. The role has been unfilled in recent years.

What triggers these ruminations is the case and controversy over the Alien Enemies Act. The high court allowed use of that law, which has stood since 1798, in removing from the country persons the government deems dangerous, though it ruled that due process must be observed. All nine justices agreed with that. The majority also moved the venue to Texas from D.C., no doubt to the consternation of the presiding judge, James Boasberg.

Justice Barrett did not join the liberals in finding the majority’s “legal conclusion’s suspect” and tantamount to a “reward” for the “Government’s efforts to erode the rule of law.” That part of the dissent, penned by Justice Sonia Sotomayor, ends with the admonition that “We, as a Nation and a court of law, should be better than this” and contends that “The Government’s conduct in this litigation poses an extraordinary threat.”

Even a partial dissent, though, is newsworthy from a justice who was nominated to the high bench by Mr. Trump. Justice Barrett’s comfort sitting in dissent in this case comes after her joining last month with the liberals — and Chief Justice Roberts — to undo Mr. Trump’s freeze on some foreign aid. That was enough to clear a majority, which prompted Justice Samuel Alito to author a fiery dissent of his own. 

Justice Barrett’s position in the foreign aid case prompted howls from MAGA-world. One influencer, Laura Loomer, even called her a “DEI appointee” because Mr. Trump had vowed that a woman would replace Justice Ruth Bader Ginsburg. Never mind that she finished first in her class at Notre Dame Law School and clerked for two conservative legal titans, Judge Laurence Silberman, of the D.C. Circuit, and Justice Antonin Scalia.

We defended the doughty justice then, reasoning that the “left’s crusade during the Biden years against the high court’s legitimacy —  joined by attacks on the integrity of the members of its conservative majority” — was in danger of replication by the right. In any event, the criticism hardly seems commensurate. Justice Barrett joined her conservative colleagues in voting to overrule Roe v. Wade, and concurred in the presidential immunity decision.

Recent Supreme Court justices — not only  O’Connor and Kennedy but also David Souter and John Paul Stevens — sometimes frustrated the legal right by migrating leftward during their decades on the court. The Framers, in ordaining that federal judges “hold their Offices during good Behaviour” — effectively a lifetime — set the circumstances for insulation from political influence.

That brings us back to the question of who, or what exactly, Justice Barrett is. She is known to be a devout Catholic, which prompted hostility from Senator Feinstein, for one, during her confirmation hearings for a lower judgeship. Some read her last interaction with Mr. Trump, at his address to a joint congress, as telegraphing hostility. Her time on the court has disclosed her to be a conservative with a maverick streak.