The Supreme Court’s decision to bestow presidents with immunity from prosecution over official actions is an extraordinary expansion of executive power that will reverberate long after Donald J. Trump is gone.

Beyond its immediate implications for the election subversion case against Mr. Trump and the prospect that he may feel less constrained by law if he returns to power, the ruling also adds to the nearly relentless rise of presidential power since the mid-20th century.

It had seemed like a constitutional truism in recent years when more than one lower-court opinion addressing novel legal issues raised by Mr. Trump’s norm-breaking behavior observed that presidents are not kings. But suddenly, they do enjoy a kind of monarchical prerogative.

“The relationship between the president and the people he serves has shifted irrevocably,” Justice Sonia Sotomayor wrote in an outraged dissent joined by the court’s other two liberals. “In every use of official power, the president is now a king above the law.”

Dismissing those worries, Chief Justice John G. Roberts Jr., writing for the majority, argued that presidents stand apart from regular people, so protecting them from prosecution if they are accused of abusing their powers to commit official crimes is necessary.

“Unlike anyone else,” he wrote, “the president is a branch of government, and the Constitution vests in him sweeping powers and duties.”

He added: “Accounting for that reality — and ensuring that the president may exercise those powers forcefully, as the framers anticipated he would — does not place him above the law; it preserves the basic structure of the Constitution from which that law derives.”

These dueling claims of what risk the founders meant the Constitution to guard against will take their place in the annals of a multigenerational argument over presidential powers.

No former president before Mr. Trump has been charged with committing crimes while president. That has raised the question of whether previous presidents were immune and the Justice Department under President Biden broke a norm by allowing a special counsel to charge Mr. Trump — or whether it was just that most other presidents were not criminals.

After Richard M. Nixon resigned to avoid being impeached for his abuses of official power in the Watergate scandal, his successor, Gerald R. Ford, shut down a criminal investigation into Nixon by pardoning him. The act was unpopular enough that it may have cost Ford the 1976 election.

Nixon accepted the pardon. But under the Supreme Court’s ruling on Monday, that whole exercise was seemingly unnecessary.

Before Nixon was forced to step down, executive power had been surging for decades. As World War II bled into the early Cold War, presidents of both parties began acting more unilaterally, especially in matters of national security, while claiming constitutional rights to keep information secret from Congress and the courts.

The historian Arthur C. Schlesinger Jr. famously described this pattern as “the imperial presidency” in a 1973 book. The surge peaked with Nixon, who later summed up his philosophy of executive power as “when the president does it, that means that it is not illegal.”

The trend briefly flagged in the mid-1970s because of Watergate, the Vietnam War and a congressional investigation that uncovered domestic intelligence abuses by administrations of both parties. In this period, Congress tried to restore checks and balances with a series of new laws and oversight actions.

But starting with the Reagan administration in the 1980s, those restraints began to erode again. Ronald Reagan and his team sought to push an activist conservative policy agenda in the face of resistance from a Congress long controlled by Democrats.

As a result, lawyers in his administration developed constitutional theories that would allow Reagan to do what he wanted even if Congress said otherwise. Among them, for example, was the so-called unitary executive theory, which says Congress cannot fracture a president’s control of the executive branch by bestowing independent decision-making authority on, say, a regulatory agency.

To be sure, Democratic presidents have pushed the envelope on discrete issues, too. Executive power often acts like a one-way ratchet: It is easier to increase than to roll back again, as one president’s innovations become a base line of precedents for his successor of either party to build upon when a perceived need arises.

But the political contingencies of the Reagan era led its push to expand presidential power to be absorbed into the conservative legal movement that was also spreading in the same period, and has come to shape ambitious Republican lawyers.

Over time, that attitude has migrated into the upper ranks of the judiciary as Republican presidents have nominated lawyers who were not just ideologically conservative but also had backgrounds in the executive branch.

Three members of the Supreme Court’s conservative supermajority — Chief Justice Roberts and Justices Clarence Thomas and Samuel A. Alito Jr. — were Reagan administration lawyers.

Two others, Justices Neil M. Gorsuch and Brett M. Kavanaugh, worked for the George W. Bush administration. It advanced a broad view of a president’s exclusive constitutional powers, especially in the sort of national security matters that frequently arose after the terrorist attacks of Sept. 11, 2001.

Only Justice Amy Coney Barrett, a former law professor, never worked as a lawyer for the executive branch. Notably, even as she joined the majority opinion, she issued a more restrained concurrence, siding with the dissent on whether prosecutors should at least be able to tell juries about a president’s official actions if they are relevant context for understanding unofficial actions he is being prosecuted for.

The structure of accountability — or lack thereof — for official presidential criminality that the country lives under now, as laid down by Chief Justice Roberts’s majority opinion, has three categories.

The first is unofficial crimes that happen to be committed by someone who is president but fall entirely outside the outer perimeter of presidential responsibilities. In theory, a former president can still be prosecuted for those types of crimes.

At the other end of the spectrum are crimes that a president commits as part of his “core” constitutional powers and responsibilities. Congress cannot intrude on how a president exercises those powers through criminal law, the majority said.

So presidents may freely abuse those powers with absolute immunity from later prosecution. At a minimum, this category clearly includes those listed in the Constitution, like granting pardons or vetoing legislation.

But the majority opinion said this category also extends to Mr. Trump’s attempt to get Justice Department officials to gin up inquiries into sham claims of voter fraud.

Chief Justice Roberts wrote that a president has “exclusive authority over the investigative and prosecutorial functions of the Justice Department and its officials.” By that measure, he said, the president “may discuss potential investigations and prosecutions with his attorney general and other Justice Department officials” under the constitutional duty to “take care that the laws be faithfully executed.”

This line was particularly notable because since Watergate, there has been a norm of Justice Department investigative independence from White House control. But Mr. Trump already eroded that norm under his administration and has openly vowed, should he return to power, that he would use the Justice Department to exact retribution upon his enemies.

Finally, the majority opinion outlined a third, more ambiguous category. This one encompasses official actions a president takes that are not core executive powers, so Congress shares overlapping authority over them and, in theory, criminal laws could apply to them.

A president “presumptively” has immunity from criminal prosecution for actions that fall in this category, too, the majority opinion said, but that shield might be overcome if prosecutors “can show that applying a criminal prohibition to that act would pose no ‘dangers of intrusion on the authority and functions of the executive branch.’”

In her dissent, however, Justice Sotomayor portrayed that purported distinction as farcical. In practice, she said, it will be essentially impossible for prosecutors to show that there is “no” danger of such intrusion.

Declaring that the majority invented a “law-free zone” around the president that will remain a “loaded weapon” for future occupants of the White House to wield, she listed “nightmare scenarios”:

“Orders the Navy’s SEAL Team 6 to assassinate a political rival? Immune. Organizes a military coup to hold on to power? Immune. Takes a bribe in exchange for a pardon? Immune. Immune, immune, immune.”

Chief Justice Roberts, in response, called that litany of possible abuses “fear-mongering on the basis of extreme hypotheticals.” The dissenting justices, he wrote, overlooked a “more likely prospect of an executive branch that cannibalizes itself, with each successive president free to prosecute his predecessors, yet unable to boldly and fearlessly carry out his duties for fear that he may be next.”

But Justice Sotomayor accused the majority of being so fixated on a president’s need for boldness and dispatch that they ignored “the countervailing need for accountability and restraint.” Never before in U.S. history, she added, have presidents had reason to believe that they would be immune from prosecution if they used their office to commit crimes.

“Moving forward, however, all former presidents will be cloaked in such immunity,” she wrote. “If the occupant of that office misuses official power for personal gain, the criminal law that the rest of us must abide will not provide a backstop.”