The Supreme Court’s landmark presidential immunity case is captioned Trump v. United States, but the first beneficiary of its ruling on presidential prerogative could prove to be none other than President Biden. That is a twist saturated with irony, given that Mr. Biden called the decision a “dangerous precedent” and “an attack” on the Constitution — and declared “I dissent,” as if he were a 10th justice. Turns out that the ruling could be a godsend.

The Nine determined that the pardon power — which Mr. Biden exercised on behalf of his son Hunter  — is part of the “conclusive and preclusive” authority of the president. The court held that all official presidential acts are presumptively immune, but that a smaller subset, including pardoning, are entitled to “absolute immunity.” The Constitution empowers the president to “grant Reprieves and Pardons for Offences against the United States.”

In oral arguments in Trump, the government’s advocate, Michael Dreeben, acknowledged that “core powers . . . can’t be regulated at all, like the pardon power and veto.” Mr. Dreeben understood that these powers could be exercised by any president, and Justice Neil Gorsuch pressed that the court was tasked with issuing a “rule for the ages.” Chief Justice Roberts wrote that “the Court cannot afford to fixate exclusively, or even primarily, on present exigencies.”

Now comes a new exigency, one that Mr. Biden insisted would never arise — a “full and unconditional pardon” for his son Hunter for “those offenses against the United States which he has committed or may have committed or taken part in during the period from January 1, 2014 through December 1, 2024.” The Supreme Court’s ruling means that not only will Mr. Biden fils be protected — so will the paterfamilias. 

If the Supreme Court had not ruled in Trump’s favor on presidential immunity, the 47th president could have investigated Mr. Biden for, say, obstruction of justice  — the crime that Special Counsel Robert Mueller contemplated against Trump during his first term. The Constitution’s plain language protects the beneficiary of a pardon from such charges — but not the president who issues the pardon. 

Now, though, Presidents Biden and Trump and future presidents cannot be investigated, let alone prosecuted, for how they dispense pardons. That is in accordance with not only the Constitution but also with the intent of its Framers. In 74 Federalist Hamilton wrote that “Humanity and good policy conspire to dictate, that the benign prerogative of pardoning should be as little as possible fettered or embarrassed.” Immunity banishes embarrassment.

Mr. Biden was so exercised by the ruling that now protects him that he proposed a constitutional amendment to reverse it, along with a whole host of other notions to cut the high court down to size. It could be, though, that his party, now out of power, will come to appreciate the Court’s rulings not only on immunity — but also in respect of, say, the overruling deference to agencies that will now be stocked with Trump’s appointees eager to take action.

We have urged Mr. Biden to pardon Trump as broadly as he pardoned his son, though Hunter Biden’s crimes were less ambiguous than those laid against Trump. Senator Manchin today urged Mr. Biden to pardon Trump. As the 46th president enjoys his retirement, free of any specter of prosecution for his act of clemency, he could spare savor the principle that what’s good for the goose is good for the blasted gander.