“This great nation can tolerate a president who makes mistakes,” declared Senator Orrin Hatch, the Utah Republican. “But it cannot tolerate one who makes a mistake and then breaks the law to cover it up.”

No, Mr. Hatch wasn’t talking about Donald Trump. It was 1999, and he was talking about Bill Clinton.

At that time, the American system — and the flawed yet sometimes heroic people their fellow Americans choose to lead them — underwent, and passed, a hard test: The president, his financial dealings and his personal relationships were painstakingly investigated for years. Prosecutors ultimately accused Mr. Clinton of lying under oath, to cover up a sexual affair. The House of Representatives impeached him, but the Senate declined to convict, and Mr. Clinton stayed in office.

The public, which learned in detail about everything investigators believed Mr. Clinton had done wrong, overwhelmingly agreed with the judgment of the Senate. It was a sad and sordid and at times distracting business, but the system worked.

Now Mr. Hatch and his fellow lawmakers may be approaching a harsher and more consequential test. We quote his words not to level some sort of accusation of hypocrisy, but to remind us all of what is at stake.

News reports point to a growing possibility that President Trump may act to cripple or shut down an investigation by the nation’s top law-enforcement agencies into his campaign and administration. Lawmakers need to be preparing now for that possibility because if and when it comes to pass, they will suddenly find themselves on the edge of an abyss, with the Constitution in their hands.

[Keep up with the state of the national debate right in your inbox by subscribing to the Opinion Today newsletter.]

Make no mistake: If Mr. Trump takes such drastic action, he will be striking at the foundation of the American government, attempting to set a precedent that a president, alone among American citizens, is above the law. What can seem now like a political sideshow will instantly become a constitutional crisis, and history will come calling for Mr. Hatch and his colleagues.

For months, investigators have been examining whether Mr. Trump’s campaign conspired with the Russian government to undermine American democracy, and whether the president misused his power by obstructing justice in an effort to end that investigation.

Until the last few weeks, Mr. Trump had shown restraint, by his standards, anyway. He and his lawyers cooperated with investigators. Mr. Trump never tweeted directly about Robert Mueller, the special counsel, and spoke about him publicly only when asked.

Alas, that whiff of higher executive function is gone. Mr. Trump is openly attacking both Mr. Mueller and Rod Rosenstein, the deputy attorney general, appointed by Mr. Trump himself. Mr. Rosenstein is overseeing the Russia investigation and signing off on Mr. Mueller’s actions.

Of course, this president has been known to huff and puff, to bluff and bluster, and he may be doing no more than that now. He may choose not to fire either man. We know he has already twice told his aides he wanted Mr. Mueller fired, only to be talked out of such rash action.

But if the president does move against the investigators, it will be up to Congress to affirm the rule of law, the separation of powers and the American constitutional order. The miserable polarization and partisan anger that have been rising in American life for decades will hit a new crescendo, and that will present congressional Republicans with a heavy burden indeed.

Mr. Trump’s Tweets on the Rule of Law

Yet if Mr. Trump goes after Mr. Mueller or Mr. Rosenstein, even Republicans who have misgivings about the president might be inclined to fall into line. They may resent what feels like an endless investigation, one that is endangering their agenda; or they may resent partisan attacks on Mr. Trump. Such frustrations — like ones Democrats vented when Mr. Clinton was in investigators’ sights — are certainly understandable. Republicans may also find themselves tempted by the political running room they would have with the investigation ended and the three branches of government under their control.

Maybe — and this is the scariest contingency to contemplate — Republican leaders would calculate that with their support, or mere acquiescence, Mr. Trump could get away with it. The overwhelming majority of Americans, including most Republicans, want Mr. Mueller to keep his job, and perhaps a groundswell of revulsion at unchecked presidential power would follow any action against the special counsel. But many Americans, weary of the shouting in Washington, might dismiss the whole thing as another food fight. We can be fairly certain that the pressure on Republican lawmakers from the minority of Americans who support Mr. Trump, as well as from the likes of Fox News and Sinclair, would be intense.

Of course, it’s when overriding your principles is the easy thing to do that you have an urgent responsibility, and opportunity, to demonstrate that you have some.

Look at what’s happening in Missouri right now. The state’s Republican governor, Eric Greitens, has been accused of sexual assault and coercion, and is scheduled to face trial next month on a felony charge of invasion of privacy. It’s a scandal of Trumpian proportions, and Mr. Greitens is responding with Trumpian bravado, calling the investigation and prosecution a “political witch hunt.”

Other Republicans On The Rule Of Law “In a country based on the system of laws, which is really the great gift given to us under the terms of our Constitution, there needs to be a consistency of application. The idea that all people are equal under the law is not a relative term.” — JUDD GREGG, 1999 “I have asked myself how men from an era when honor was valued above all other traits, men like George Washington, Thomas Jefferson, John Adams and James Madison, might have viewed a President who committed perjury and obstruction of justice for personal and political gain.” — Phil Gramm, 1999 “What standard of conduct should we insist our President live up to? ... Do not underestimate, my friends, the corrupting and cynical signal we will send if we fail to enforce the highest standards of conduct on the most powerful man in the nation.” — Pete Domenici, 1999 “Committing crimes of moral turpitude such as perjury and obstruction of justice go to the heart of qualification for public office.” — Orrin Hatch, 1999

Yet the legislative report detailing his misbehavior was bipartisan, and top state Republicans have spoken out forcefully. They recognize that Mr. Greitens is unfit. (They also see a threat to their political interests, but the two can go hand in hand.)

Or look at Watergate. We may think of it now as a two-year drama with an inevitable end, the takedown of a president who tried to cover up a criminal conspiracy. But many people forget how close President Richard Nixon came to surviving the affair. He was forced from office only because enough Republican leaders recognized the legitimacy of the investigation and stood up to him. And even then, it took the revelation of incriminating recordings. No recordings have come out this time — yet.

A few senior Republicans have been saying the right things — including Mr. Hatch. He tweeted that anyone telling the president to fire Mr. Mueller “does not have the President or the nation’s best interest at heart.” Senator Lindsey Graham, of South Carolina, warned Mr. Trump that firing Mr. Mueller would be “the beginning of the end of his presidency.”

That’s all necessary and good. But it’s not enough. More Republicans need to make it clear that they won’t tolerate any action against either man, and that firing Mr. Mueller would be, as Senator Charles Grassley said, “suicide.”

Mr. Mueller’s investigation has already yielded great benefit to the country, including the indictments of 13 Russians and three companies for trying to undermine the presidential election. None of us can know if prosecutors will eventually point the finger at the president himself. But should Mr. Trump move to hobble or kill the investigation, he would darken rather than dispel the cloud of suspicion around him. Far worse, he would free future presidents to politicize American justice. That would be a danger to every American, of whatever political leaning.