During the last two days of his embattled presidency, Richard Nixon made a rare principled decision. With the Watergate special prosecutor and congressional impeachment proceedings closing in, he rejected last-minute requests for pardons from his two former top aides, the men who could most damagingly testify against him. Unfortunately, while dust is settling on Donald Trump’s pardon of former Arizona sheriff Joe Arpaio, its message suggests a much cruder view of the pardon power, and sets a dangerous precedent for the months to come.

Most pardons attract little attention. Over the course of a presidency, typical presidents, with the advice of the Department of Justice, pardon or commute the sentences of hundreds of people who have moved beyond their crimes, and long after all investigations have ended. Sometimes, presidents issue pardons to try to help the nation move past bitter divisions, such Gerald Ford’s pardon of Nixon and Jimmy Carter’s general pardon of Vietnam draft dodgers. And occasionally, presidents issue pardons to friends, relatives, or supporters, and even at the request of a foreign government, such as Bill Clinton’s controversial pardon of fugitive financier Marc Rich.

But never has a president embroiled in scandal used the pardon power to protect himself. Nixon’s refusal provides useful history. On March 1, 1974, a federal grand jury indicted former chief of staff H.R. Haldeman, former chief domestic advisor John Ehrlichman, and five other aides for criminal conspiracy stemming from the Watergate cover-up. The president was exposed as well. To be sure, the Department of Justice took the position that a president cannot be indicted while in office. But the grand jury named Nixon as an unindicted co-conspirator. And before Ford pardoned him, he risked future prosecution.

On August 7, Haldeman and Ehrlichman, employing different channels, approached the White House and asked for pardons. They did not need to point out that the risk of prison time makes people more likely to testify. Yet Nixon rejected their request. The next day, he announced his resignation and submitted himself to fate. Gerald Ford later pardoned Nixon on September 8, 1974. Haldeman and Ehrlichman each served 18 months in prison. Neither ever fully testified under oath; their accounts of the Watergate cover-up are found only in their White House recordings, and Nixon’s secret tapes.

Later presidents have also declined to use the pardon power to protect themselves from scandal. George H.W. Bush, who may have faced some legal risk from the Reagan administration’s Iran-Contra scandal, eventually pardoned several key figures — but long after anyone might have had an incentive to implicate him. Bill Clinton pardoned his former law partner, Susan McDougal, of charges stemming from the Whitewater deal — but only after she had already been released from prison.

Arpaio’s pardon sent a signal that reverberated in Washington. For Trump’s current and former associates embroiled in Special Counsel Robert Mueller’s Russiagate investigation, the message is unmistakable: don’t cooperate, and I’ll pardon you.

First, the timing: Most presidents use clemency cautiously; “hot” pardons, if addressed, tend to come towards the end of a president’s final term. The fact that Trump is willing to pardon and praise a polarizing figure like Arpaio after just seven months in office is telling. It indicates that he will not likely worry about political blowback from pardoning former subordinates who could implicate him in wrongdoing, whenever he feels it is necessary.

Second, the crime: Arpaio was convicted of contempt of court for disobeying a court order. The pardon signals to Trump’s current and former aides that, if they are threatened with contempt for refusing to testify, he will pardon them too. This president is not troubled by the separation of powers.

Third, the motive: There are not many other options for Trump to convey that message to former aides, such as former campaign chairman Paul Manafort and former national security advisor Michael Flynn, who are in the special counsel’s sights.

If he tried to reassure them directly, it could constitute witness tampering or other obstruction of a criminal investigation, in violation of federal law. In the Nixon impeachment proceedings, Congress specifically cited Nixon’s various promises of executive clemency to induce various Watergate participants to plead guilty and refuse to testify.

But one effective way to send the message that “if the heat gets too close, hold tight and I will pardon you” is to pardon someone else. Ideally, someone just as controversial — someone like Arpaio. That gives the president plausible deniability. Manafort, Flynn, and other Trump associates understand the signal, but the president can deny that he encouraged them not to cooperate with law enforcement.

The Framers were aware of this danger. In the Virginia constitutional ratification debates, George Mason criticized the pardon power, arguing that the president “may frequently pardon crimes which were advised by himself,” and so “destroy the republic.” James Madison replied that if there were grounds to believe that the president would use the pardon to “shelter” a person with whom he was “connected, in any suspicious manner,” then Congress could impeach him.

Federal courts can also restrain a president’s abuse of the pardon power. To be sure, the Supreme Court has not held a presidential pardon invalid since 1915, and under different circumstances. But the Justices have hinted from time to time that at least some clemency decisions might be open to challenge, and advocates are making solid legal arguments about why the Arpaio pardon crosses a constitutional line.

Coming months will be critical. Mueller appears to be moving aggressively — just ask Manafort and Flynn. He has advised the White House that he plans to interview six former and current aides. As the investigation closes in, the incentives for cooperation will increase, and so will the temptation for Trump to try to make it all go away. Before that happens, responsible members of Congress, the media and public should make clear that abuse of the pardon power by Trump to impede Mueller’s investigation will have the same consequences as firing the special counsel: Impeachment proceedings will commence immediately.

Sign up for Inside TIME. Be the first to see the new cover of TIME and get our most compelling stories delivered straight to your inbox. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Contact us at letters@time.com.