Almost immediately after Justice Anthony Kennedy announced his retirement on Wednesday, Senate Democrats argued that his replacement should not be confirmed until after the midterm elections this fall — a version of the same argument that Mitch McConnell, the Senate majority leader, used to stymie President Barack Obama’s nomination of Merrick Garland to the Supreme Court in 2016.

This is surely a valid argument, not least because Mr. McConnell’s blatantly anti-democratic ploy stole a judicial appointment from a popularly elected president and gave it to one who lost the popular vote by millions.

But there is another reason to withhold confirmation that both Republicans and Democrats should be able to agree on: People under the cloud of investigation do not get to pick the judges who may preside over their cases. By this logic, President Trump should not be permitted to appoint a new Supreme Court justice until after the special counsel investigation is over, and we know for sure whether there is evidence of wrongdoing.

True, that point is unlikely to stop Mr. McConnell or his colleagues. But it highlights the real risk involved in letting a deeply compromised president shape a court that may one day stand between him and impeachment.

Much of the conversation since Justice Kennedy announced his retirement has been focused on whether a more conservative replacement might lead to the overthrow of landmark decisions on abortion rights, gay marriage and other issues. These are undoubtedly important concerns. But not enough attention has been placed on the crucial question of whether the Supreme Court in the Trump era will provide an effective bulwark against autocratic lawless rule.

Indeed, legal experts are already debating several knotty constitutional questions that involve the president and may one day soon have to be decided by the court. Can the president pardon himself or others specifically to extricate himself from criminal investigation? Can the president be compelled to testify before a grand jury? Can a sitting president be criminally indicted?

Did the appointment of the special counsel somehow violate the Appointments Clause of the Constitution, as some conservatives implausibly insist? Can a president ever obstruct justice? What is the proper legal remedy for Mr. Trump’s repeated violations of the Emoluments Clause? It is no exaggeration to say that never before has the selection of a Supreme Court nominee been so thoroughly compromised by the president’s profound personal interest in appointing a judge he can count on to protect him.

While we cannot know how Justice Kennedy would have ruled on these questions, we do know that at least at times he was willing to stand up to assertions of power by the executive branch, most notably in Boumediene v. Bush, when he wrote a 5-4 decision defying the president and extending the constitutional right of habeas corpus to wartime detainees at Guantánamo Bay.

Mr. Trump’s possible crimes are inextricable from his desire for unilateral control of the federal government. It is no secret that the power of the executive branch has grown over the past several decades, under both Republican and Democratic presidents. Our executive now has surveillance capacities never before seen, vast power to conduct drone strikes and conduct lethal military operations abroad, broad authority to set immigration and law enforcement priorities and the ability to regulate enormous areas of economic and personal life.

Add to this sweeping institutional power a president who refuses to acknowledge any checks on his power as legitimate, whether those checks come from the courts, the legislature, the media, the government bureaucracy or his political opponents. This is the perfect recipe for autocracy. In such a world, the importance of checks and balances has never been greater.

This would be dangerous regardless of Mr. Trump’s legal shortcomings. But this president has, by his own admission, already taken steps to thwart an investigation into his own potential criminality. Both Democrats and Republicans in the Senate should therefore resist calls for a quick confirmation process.

Otherwise, there will be a stain on the legitimacy of this nomination, on the performance of whomever is confirmed and, even, on the Supreme Court itself. The fact that the president has every motive to ensure that happens — to promote his political agenda and to protect him personally — makes the present moment all the more frightening.