The Supreme Court handed President Obama his 13th unanimous loss in two years on Thursday, and this one may be the most consequential. All nine Justices voted to overturn Mr. Obama’s non-recess recess appointments as an unconstitutional abuse of power.

Over nearly 238 years of American history, the Supreme Court has never had to review the President’s authority to temporarily fill vacant executive offices when Congress is adjourned. Mr. Obama’s 2012 maneuver to void the Senate’s advice and consent role triggered a judicial intercession, and defeats at the High Court are seldom as total as this one.

Two years ago Mr. Obama packed the National Labor Relations Board with three new members and made Richard Cordray the chief of the new Consumer Financial Protection Bureau. Other Presidents have made such appointments and we’ve long supported that authority—as long as they are made when Congress is genuinely in recess.

But in this case the Senate was conducting pro forma proceedings (gavel in, gavel out, every three days) because neither chamber can adjourn without the other’s permission under Article I, Section 5. The House refused to consent to prevent Mr. Obama from making recess appointments, so he simply assumed the power to define on his own when a coequal branch of government is at work.

On this invention, the President could presumably make recess appointments overnight or during a lunch break, but Mr. Obama’s provocation was deliberate. “I refuse to take no for an answer,” he justified his behavior at a campaign event the day after the appointments. Democrats ran the Senate then and run it now. Mr. Obama merely thought the normal confirmation checks and balances too frustrating and preferred to install his union appointees without a debate.

He should have read the Recess Appointments Clause before Justice Stephen Breyer did it for him. In Noel Canning v. NLRB, a Washington state soda bottler challenged a board decision on grounds that the recess appointments were null and thus the board lacked the three-member quorum to do business. Because the Constitution delegates power to each branch to independently make their own rules, writes Justice Breyer, “the Senate is in session when it says it is.”

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Justice Breyer surveys the legal theories and evolution over time of recesses and recess appointments since the Federalist Papers. This exception to advice and consent was necessary because Members of the early Congresses were out of town for months at a time, while the executive branch was so small that a few job openings could shut down the government. In the 20th century, recess appointments became more common even as their original purpose disappeared.

Deferring to this historical practice, Justice Breyer arrives at a pragmatic test: The President may fill vacancies when the Senate has not transacted business for 10 days or more, whether within or between Congress’s two year-long formal sessions.

The Constitution lacks any such 10-day clause, and it is troubling that Justice Breyer seems to have invented it on his own. But still his invention narrows the recess power. And had Justice Anthony Kennedy flipped and joined the four conservatives, recess appointments would have been diminished even more.

In a concurrence with the judgment only, Justice Antonin Scalia makes a stricter reading of the Constitution’s language about “vacancies that may happen during the recess.” His interpretation would limit recess appointments to only the break between formal sessions and only for positions that open during that window. Justice Breyer “casts aside the plain, original meaning of the constitutional text in deference to late-arising historical practices that are ambiguous at best,” he writes.

We admire Justice Scalia’s originalism, but the clause is ambiguous; Thomas Jefferson puzzled over its meaning as early as 1802. But Justice Scalia’s reasoning shows why Mr. Obama’s gambit was so reckless. “Friction between the branches is an inevitable consequence of our constitutional structure,” Justice Breyer instructs, and the legislature and executive are supposed to work things out along the way. By violating these norms, Mr. Obama invited the judiciary to mediate and jeopardized the recess power for all future Presidents.

The Framers did not vest the executive with the unilateral appointment authority that Mr. Obama thinks he is entitled to. They wanted to diffuse power across the federal government to protect individual liberty. Wilfully bypassing advice and consent also subverts political accountability, which a former constitutional law professor ought to know.

Mr. Obama has thus strengthened the Senate, now armed with a judicial guide to preventing recess appointments: Presidents must take no for an answer. The ruling also opens to challenge some 436 decisions that the NRLB issued while the imposter members were seated.

But the true import of Noel Canning is that even liberal Justices are alarmed that Mr. Obama’s executive law-making is visiting real damage on the Constitution. This will not be the last legal torpedo aimed at the hull of his increasingly willful Presidency.