Near the end of his opinion on executive immunity, Chief Justice John G. Roberts Jr. pooh-poohed the fears of his liberal colleagues who worried in dissent that the broad protections the Supreme Court had conferred on former President Donald J. Trump would place future presidents beyond the reach of the law.

The real concern, Chief Justice Roberts said, was not that immunity would embolden presidents to commit crimes with impunity, but rather that without it, the country’s rival leaders would endlessly be at each others’ throats.

“The dissents overlook the more likely prospect of an executive branch that cannibalizes itself,” he wrote, “with each successive president free to prosecute his predecessors.”

That dark vision, however right or wrong it proves to be, did not come out of nowhere: It was offered to the court by Mr. Trump’s own lawyers during oral arguments on the question of immunity that took place in April.

The justices in the majority said their decision was not just about Mr. Trump. But it was impossible to separate it from the possibility of a second Trump presidency following a campaign in which Mr. Trump himself has promised unabashedly to use the legal system as a weapon of political retribution against President Biden and other foes, whom he accuses of having unfairly targeted him for prosecution.

In many ways, the court’s decision was something like a Rorschach test for the justices, revealing what they saw as the largest looming threat to American democracy.

For the conservatives, that threat is the prospect of ceaseless cycles of partisan prosecutions constraining a president’s ability to make decisions in the best interests of the country.

The main concern, the chief justice wrote, was to insulate a president from the perils of “hesitation to execute the duties of his office fearlessly and fairly” because of a fear of prosecution.

The liberals, by contrast, feared a monarchical president who could use the immense powers of the office for personal or political gain or for other illegitimate purposes without the legal checks and balances that they say have long been necessary to ensure accountability.

Overall, the ruling reflected just how far the court’s conservative majority, which Mr. Trump helped to expand, has gone in endorsing his view of politics as a no-holds-barred battle for power and his contention that American leaders will, as a matter of course, now seek to prosecute their rivals.

Trevor Morrison, a professor at the New York University School of Law, agreed that the court’s decision focused a spotlight not only on what the justices feared about the future of the country, but also on the central principles they held as federal jurists.

“The two sides differ greatly in what they saw as the chief danger that each wants to protect against,” he said. “But you can also contrast their values, with the majority wanting the presidency not to be undermined by constant prosecutions and the dissenters showing concern about ensuring the rule of law.”

These two different views about the nature of politics and power were far more than a mere philosophical dispute among the justices. The way in which the court decided the immunity case could soon have real effects on the ground.

Mr. Morrison, for example, imagined what might happen if Mr. Trump were re-elected and appointed a pliant attorney general to go after Mr. Biden.

“The majority on the court would likely say that Biden enjoys a wide sweep of immunity from prosecution,” Mr. Morrison said. “But at the same time, no one would be able to go after Trump for weaponizing the Justice Department.”

Mr. Trump’s lawyers first presented their idea that in the future, former presidents will be relentlessly pursued by their successors in written filings to the court. But their fullest articulation of that stance came during the court’s oral arguments.

That was when D. John Sauer, who argued before the justices on Mr. Trump’s behalf, asked them to imagine someone — Mr. Trump was not identified by name — prosecuting Mr. Biden for his immigration agenda.

“Could President Biden someday be charged with unlawfully inducing immigrants to enter the country illegally for his border policies?” Mr. Sauer asked.

Answering his own question, Mr. Sauer envisioned a world of constant legal tit-for-tat, asserting that it would destroy the “presidency as we know it.” And several of the court’s conservatives appeared to agree that world was on its way — or perhaps already here.

Justice Samuel A. Alito Jr., for example, expressed concern that without some form of criminal immunity, former presidents would be highly vulnerable as their successors used the courts to go after them once they were out of office. And that, he added, could lead to persistent cycles of retribution that would present a risk to a “stable, democratic society.”

Mr. Trump has long — and baselessly — maintained that it is Mr. Biden and his Democratic allies who politicized justice by pursuing him with multiple indictments even as they have faced each other on the campaign trail. But in advancing those claims, Mr. Trump has never acknowledged the reality that no other president has been faced with as many allegations, or as much evidence, of wrongdoing as he has.

In its ruling, the Supreme Court’s majority tacitly bought into Mr. Trump’s portrayal of himself as a victim of partisan legal warfare — if only by endorsing his vision of a world where presidents would ruthlessly use their powers to go after their predecessors.

That bleak view was also in keeping with another of the former president’s longest-held beliefs: that there are no good guys in the world because everyone is corrupt.

Against that dystopian vision, the liberal dissenters, particularly Justice Sotomayor, asserted the idea that immunity was unnecessary to protect a former president against partisan indictments since the job could effectively be handled by “all the protections our system provides to criminal defendants.”

Justice Sotomayor reminded the conservative majority that it was not exactly easy to indict a former president — a proposition that seemed in keeping with the amount of time and angst it required to bring charges against Mr. Trump and the subsequent difficulty prosecutors have had in advancing those cases.

There were built-in checks and balances, Justice Sotomayor went on to say, to stop a prosecutor from bringing cases without merit. She cited the grand jury process, for example, the ability to file motions to dismiss and the burden of proof that prosecutors must assume at trial.

But Chief Justice Roberts seemed to play down the suggestion that any normal legal roadblocks could slow a partisan prosecutor on a mission to indict a political rival, claiming that immunity was needed because, by its very nature, it stopped prosecutions from ever reaching a courtroom.

He also seemed to reject the Justice Department’s assurances — echoed by the dissenting liberals — that “prosecutors and grand juries will not permit political or baseless prosecutions from advancing in the first place,” as he put it.

“We do not ordinarily decline to decide significant constitutional questions,” he wrote, “based on the government’s promises of good faith.”