The intimidation game has begun. President Biden announced last Friday the formation of a commission on reforming the Supreme Court, and Democrats in the House and Senate responded on Wednesday by announcing that a bill to add four justices to the high court is forthcoming. These are dangerous developments. Even if court packing ultimately fails, the effort to intimidate the Supreme Court will have dire consequences.

Progressives have long used threats to pressure the Supreme Court to back down from the Constitution. In 1906 President Theodore Roosevelt’s allies hinted that the court shouldn’t reject expansion of the Interstate Commerce Act. According to my colleague Thomas Merrill, the “implied threat” included the possibility of “stripping the Court of jurisdiction over ICC matters or creating a specialized court.”

Such threats soon became standard practice. In 1909 commissioner Charles Prouty cautioned judges that if they questioned the ICC’s authority they would face popular protest that “would not stop until the Constitution itself had been so altered as to enable the people to deal properly with these public servants.” In 1912, running as the Progressive Party’s presidential nominee, Roosevelt campaigned on the recall of uncooperative judges and decisions. President Franklin D. Roosevelt went further and sought legislation to pack the Supreme Court.

Progressives never had to carry out their threats to get what they wanted. All they had to do was encourage judges to consider the advantages of judicial “restraint” and “deference” to the political branches. Unnerved by the bullying, some Supreme Court justices lost their courage. By holding back from overturning popular legislation, they thought they could “save the Court.”

In 2012’s NFIB v. Sebelius, Chief Justice John Roberts acknowledged the high court’s “reticence to invalidate the acts of the nation’s elected leaders.” It looked as if he took this “reticence” so far as to rewrite an unconstitutional statute, lest the court provoke progressive wrath. Yet his retreat encouraged the view that the court is open to political pressure. In flinching from their duty to follow the law, judges invite further intimidation.

Jurists have a duty to exercise their own independent judgment to say what the law is. They therefore cannot refrain from holding an unconstitutional act void in response to their anxieties for the fate of the court. This is as bad as going beyond the law to hold a statute void. Both the retreat and the excess of zeal would depart from their duty.

Even if they could save their institution by doing such things, what is the point of saving the court at the cost of the law? And in what sense would the court be saved? There is little value in a court that has been corrupted by fear.

Only by standing on their office of independent judgment can judges preserve their reputation for integrity. This is the only way to preserve the ideal and the reality of an independent bench.

At this point, after more than a century of progressive threats, the justices have backed away from much of the Constitution. Limited congressional powers? Blown away with the expansion of the commerce, taxing and spending powers. Federalism? Destroyed in the same way. Legislative power vested in Congress? No longer entirely vested there, as much of it has been moved to administrative agencies. Judicial power vested in the courts? Also now largely relocated to administrative agencies. Juries and other procedural rights? Increasingly irrelevant, as the government now brings most enforcement proceedings in agencies.

What about the Constitution’s substantive rights—notably those of speech, religion, assembly, guns and takings? They already are seriously eroded and will suffer further with more intimidation.

The threat of court packing is vandalism on a scale far beyond the destruction of statues. It undermines judicial duty and independence. It destroys the legitimacy of the Supreme Court and its decisions. Such threats have already demolished much of the Constitution. Will they now smash what is left?

Whatever happens, the justices have received their warning: “Bend your judgment politically, or we will politicize your court.” The damage could be irreparable. The intimidation game is being played once again, with profound costs for the Constitution and our freedom.

Mr. Hamburger is a professor at Columbia Law School and president of the New Civil Liberties Alliance.