The Supreme Court’s radical decision handing the president broad immunity from criminal prosecution on Monday will rightly be understood as enormously increasing the power and enormously reducing the accountability of the president.

But it should also be understood as a decision about the court’s own power and accountability. In casting aside the text, structure and history of the Constitution in favor of gauzy concerns about the need to “safeguard the independence and effective functioning of the executive branch” and to “enable the president to carry out his constitutional duties without undue caution,” the court reveals that it will rule — and rule us all — based on its own free-floating and distorted vision of an optimal constitutional order.

It is increasingly clear that this court sees itself as something other than a participant in our democratic system. It sees itself as the enforcer of the separation of powers, but not itself subject to that separation.

Most immediately, the decision continues to shield Donald Trump from meaningful accountability for his actions before and on Jan. 6, 2021. The court had already given Mr. Trump a decisive win in the form of its monthslong delay in deciding this case — his federal criminal trial for election interference, originally scheduled to begin on March 4, appears less and less likely ever to come to pass.

But the opinion itself grants Mr. Trump a more enduring win, and democracy an even more enduring loss: It jettisons the long-settled principle that presidents, like all others, are subject to the operation of law, and announces that all official acts taken by a president are entitled to either absolute or presumptive immunity from criminal prosecution.

The court’s misguided decision in this case could not come at a more dangerous time. It has removed a major check on the office of the presidency at the very moment when Mr. Trump is running for office on a promise to weaponize the apparatus of government against those he views as his enemies.

As a searing dissent from Justice Sonia Sotomayor makes clear, some of the more extreme hypotheticals from the oral arguments in this case are no longer purely hypothetical. She writes that this newly fashioned presidential immunity “lies about like a loaded weapon” (quoting from a dissent in an earlier decision, Korematsu v. United States) for any president who “wishes to place his own interests, his own political survival or his own financial gain above the interests of the nation.” A president “orders the Navy’s SEAL Team 6 to assassinate a political rival? Immune. Organizes a military coup to hold onto power? Immune. Takes a bribe in exchange for a pardon? Immune. Immune, immune, immune.”

It is one thing for an aspiring president to promise lawlessness. The only way to deliver on that promise is with a willing and cooperative court. It appears that Mr. Trump has one.

Almost exactly 50 years ago, in July 1974, the court rejected an extravagant claim of presidential immunity: Richard Nixon’s effort to have the court excuse him from producing White House tapes in conjunction with the special prosecutor’s investigation of the Watergate break-in and cover-up. Nixon never faced criminal charges for his involvement in Watergate because following his resignation Gerald Ford granted him a “full, free and absolute pardon,” an act Justice Brett Kavanaugh lauded during the oral argument in Mr. Trump’s case.

But the text of Ford’s pardon made clear that but for the pardon, Nixon might have faced criminal prosecution for his conduct while president. Ford wrote in his proclamation that “Richard Nixon has become liable to possible indictment and trial for offenses against the United States,” and explained that he was granting the pardon in order to avoid “exposing to further punishment and degradation a man who has already paid the unprecedented penalty of relinquishing the highest elective office of the United States.”

As the Ford-Nixon pardon made crystal clear, 50 years ago, the country understood well that ex-presidents could face trial for their crimes in office. That understanding endured until this week. Just three years ago, when Mitch McConnell voted to acquit Mr. Trump following an impeachment trial for incitement of insurrection, Mr. McConnell explained: “We have a criminal justice system in this country. We have civil litigation. And former presidents are not immune from being held accountable by either one.”

On Monday, the court radically refashioned the settled understanding of the accountability of ex-presidents.

But this case is about the court as much as it is about the president. That is because so profound a warping of the separation of powers cannot remain contained. The court in this case announces that an important mechanism of accountability, criminal charges under statutes passed by Congress, is almost entirely unavailable in the context of former presidents.

A court willing to declare the president beyond the reach of the law, regardless of the actions of either the president or Congress, could use that same power to cloak itself in the same immunity from legal process.

Although none of the court’s precedents supports this week’s immunity decision, the court does draw on a different body of reasoning, which was on display in the recent statements of justices faced with being held to account by outside entities, in particular Congress. The most egregious of these was Justice Samuel Alito’s statement last July to The Wall Street Journal about Congress and the court: “I know this is a controversial view, but I’m willing to say it,” Justice Alito said. “No provision in the Constitution gives them” — meaning Congress — “the authority to regulate the Supreme Court — period.”

Sub in “president” for Supreme Court, and that’s Monday’s opinion in a nutshell.

The court’s reasoning here is also in line with what Chief Justice John Roberts said to the Senate when the Judiciary Committee leadership wrote to him in May after the revelations that “Stop the Steal”-linked flags had been flown over Justice Alito’s homes. Among other things, the letter requested a meeting to discuss Supreme Court ethics; Chief Justice Roberts’s brusque refusal invoked broad “separation of powers concerns” that he claimed “counsel against such appearances.”

It is now clear that the Roberts court believes the separation of powers means that both presidents and courts stand beyond the reach of the law.

Although the Ford pardon meant that Nixon never stood trial, many members of the Nixon administration did; well over 40 individuals were convicted for their involvement in Watergate. But the very pardon power that Ford deployed to spare Nixon is a power Mr. Trump could weaponize in order to protect his subordinates.

The combination of this new judicially invented presidential immunity and the longstanding pardon power means that a future Trump White House could become the site of a criminal enterprise that would make Richard Nixon’s plumbers unit look like child’s play.