Andrew Harnik / AP

The most surprising thing about Wednesday’s resignation of Jeff Sessions is that it did not happen sooner. President Trump has long been furious with his attorney general for recusing himself from the Russia investigation, and has had the power to fire him at any time. Trump’s decision to oust Sessions also removes Rod Rosenstein as acting attorney general for the purposes of the Russia investigation, leaving the fate of Special Counsel Robert Mueller uncertain. This drives home an essential truth about all independent investigations of the presidency: Their outcomes aren’t determined by the prosecutors or evidence alone, but by the public’s reaction to what it learns, and to how presidents choose to react.

Americans are deeply committed to the maxim that no one, not even the president, is above the law. Even the most expansive proponents of executive power do not dispute this bedrock principle, which distinguishes constitutional democracy from monarchy. Yet no law required Rosenstein, as acting attorney general, to appoint an outside prosecutor when credible evidence emerged that the president’s 2016 campaign may have colluded with the Russian government. When Rosenstein elected to appoint such a prosecutor voluntarily, no law barred Trump from shuttering the investigation to protect his family, his friends, or himself.

Special prosecutors like Mueller serve as catalysts for democracy. Their high visibility makes it easier for the American people to monitor presidential misconduct and to hold the president accountable for his actions. But special prosecutors, no less than presidents, can misuse their power. To temper this danger, Trump retains the authority to dismiss Mueller at any time. If he exercises that power for a corrupt purpose or out of simple caprice, Mueller has no legal recourse. But he is not totally defenseless. Trump must ultimately answer to the American people. This can be a potent check, but it is also a fragile one.

President Ulysses S. Grant appointed the first special prosecutor in 1875 to investigate a corruption scandal that threatened to engulf his administration. Ever since, both Democratic and Republican presidents have appointed special prosecutors and permitted them to operate with unusual independence from presidential oversight. In short order, this became a standard method for quelling political scandals. By appointing an outside prosecutor, the president could signal confidence in his own innocence and reaffirm his commitment to the rule of law.

This long, mostly forgotten history holds important lessons. Special prosecutors can do much to safeguard the rule of law under the right conditions. They are also fallible. Many have been stymied by the formidable difficulties of investigating a sitting president and his friends, whose political incentives change once a special prosecutor has been appointed. Grant testified as a character witness on behalf of his beloved and corrupt chief of staff Orville Babcock, thwarting Babcock’s prosecution for bribery. Henry McGrath, an attorney general under Harry Truman, outright refused to answer the questions of Special Prosecutor Newbold Morris, who was investigating corruption in the tax division of the Justice Department. The special prosecutors investigating Teapot Dome, the 1920s-era corruption scandal, were forced to pay expenses out of their own pockets for long stretches.

A few special prosecutors, such as Bill Clinton–era Independent Counsel Ken Starr, have lost their sense of proportion or been carried away by monomaniacal zeal. Yet this is the exception. Far more frequently, the specter of a power-mad special prosecutor has operated as a weapon of presidential propaganda with little foundation in fact. President Trump’s ceaseless cries of “witch hunt” are but a hyperbolic echo of the supporters of Grant, Clinton, Calvin Coolidge, Ronald Reagan, and Richard Nixon.

The most important lesson of this history is that special prosecutors are incapable of saving us from ourselves. If Trump thinks he can fire Mueller without suffering serious political fallout, he has the power to do it. Three previous presidents have exercised this power. Grant and Truman, both lame ducks nearing the end of their term, escaped without serious consequence, though their parties suffered electoral losses. Nixon lived to regret it.

Ultimately, only the American people can decide whether the president is above the law. In the present circumstances, this question may seem like a purely partisan one. Those who like Trump will be tempted to view legal restraints on his power as a cumbersome nuisance. Those who dislike him will take the opposite view. All Americans, however, have a profound stake in preserving the “government of laws and not of men” passed down to us by previous generations.

The alternative is a system in which the president is “the only one that matters,” to quote Trump’s revealing self-description. That sort of despotism is just what the American founders fought a revolution to overthrow. It is also what many of our ancestors fled their home countries to escape.

Of course, special prosecutors are not the only actors in our constitutional system charged with safeguarding the rule of law. Many other institutions play important roles—or can do so, if they choose. These include Congress, the courts, and even the federal bureaucracy. But these institutions are not self-activating. If they are to perform their intended function, the people need some kind of early warning system, so they know when to press their elected representatives and other institutional checks into action. Special prosecutors can play this role very effectively, but they require popular—and, to at least some extent, bipartisan—support.

This might seem like an impossibly tall order in today’s polarized political climate, but there is a close historical precedent. In 1972, Richard Nixon was reelected by what was then the largest popular vote margin in American history. He won 18 million more votes than George McGovern and began his second term with an approval rating of 67 percent. Yet when Nixon fired the first Watergate special prosecutor, Archibald Cox, in October 1973, the public reaction was so swift and ferocious that Nixon was forced to appoint another genuinely independent special prosecutor, Leon Jaworski, within a week.

It was Jaworski’s investigation that ultimately forced Nixon’s resignation in August 1974. By that time, Nixon’s public approval had fallen to 24 percent. In other words, tens of millions of Nixon voters had changed their minds about him because of the evidence unearthed by the Watergate special prosecutors. What has happened before can happen again.

Whether Trump pays a similar price for ousting Sessions will go a long way toward determining Mueller’s fate. In the end, it is up to the American people, who will get the democracy they deserve.

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