SINCE FORMING a government in 2015, Poland’s right-wing populist Law and Justice party has systematically sought to neuter checks on its power, especially by the judiciary. Early on, it seized control of the Constitutional Tribunal; later, it revamped the system for choosing judges to give the party control, rather than judges themselves. Under pressure from the European Union, it has retreated from some steps, such as purging the Supreme Court. Yet now, having won reelection in October with 43 percent of the vote, Law and Justice is making another push to quash judicial independence.

A law hastily passed by the lower house of the Polish Parliament, the Sejm, on Dec. 20 is being called the “muzzle act” because it would provide for the fining or firing of judges who criticize the government’s actions against the judiciary or engage in other unspecified “political activities.” But “muzzling” understates the measure’s sweep. It is meant to reverse a Supreme Court ruling saying that the Law and Justice-controlled body created to appoint judges is not independent, a decision that was itself based on a judgment by the European Court of Justice, the European Union’s highest court.

Under the proposed law, which Law and Justice rammed through the Sejm over strong E.U. objections, Polish judges would not be able to overrule Polish laws that conflict with E.U. law or the constitution without approval from the Law and Justice-dominated Constitutional Tribunal. The respected head of the Supreme Court, Malgorzata Gersdorf, warned that the result would be “the liquidation of independent judicial power.” But never mind: The legislation also changes the procedure for picking Ms. Gersdorf’s replacement so that Law and Justice controls that as well.

The legislation is now before the opposition-controlled Senate, which could revise or reject it. But because any such action could be reversed by the Sejm, the measure’s enactment is likely unless Law and Justice and its strongman, Jaroslaw Kaczynski, reverse course. That doesn’t seem likely: Mr. Kaczynski recently pilloried judges he said were “evil.”

Poland is already the subject of an E.U. sanction procedure for its assault on the judiciary, and the latest act has prompted predictions that Law and Justice will drag the country out of the union altogether. Yet from Brussels’s point of view, a worse outcome is possible: that Poland dodges sanctions while remaining inside the E.U. It and Hungary could constitute a de facto authoritarian caucus, using E.U. rules to defend each other from punishment.

Poland consequently could pose a crucial test for Ursula von der Leyen, the new head of the European Commission. She ought to push a reform linking E.U. funding to compliance with the rule of law. The Central Europeans could find it harder to block that step — and Poland is the largest net recipient of E.U. subsidies paid to lower-income members.

Though contemptuous of the E.U., Mr. Kaczynski is deferential to the Trump administration, which has increased U.S. troop levels in Poland. President Trump is no champion of the rule of law, but Congress could condition military cooperation on Poland’s compliance with democratic norms. Unless the “muzzle act” is scrapped, it should.

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