“The President is to have the laws executed,” wrote the Chief Executive. “He may order an offence then to be prosecuted,” but if he “sees a prosecution put into a train which is not lawful, he may order it to be discontinued.”

That’s not a tweet from Donald Trump. It’s a letter from Thomas Jefferson, in 1801, explaining the President’s broad authority to supervise and control federal criminal prosecutions.

As special counsel Robert Mueller’s investigation heats up, Jefferson is a better guide to presidential prerogatives than the Trump critics who decry his Executive power claims as reminiscent of King George. For better and worse, the Constitution gives the President extraordinarily broad authority over investigations, pardons and firings.

The President’s pardon power is practically absolute for federal offenses. It “cannot be modified, abridged, or diminished by the Congress,” the Supreme Court has ruled. The President can issue a pardon before prosecution, and to a broad class of people. It’s unclear whether Trump can pardon himself. But he could, if he wanted, pardon everyone else Mueller is investigating, just as George H.W. Bush in 1992 pardoned six people under investigation by an independent counsel.

Trump could also order Deputy Attorney General Rod Rosenstein, Mueller’s supervisor, to dismiss Mueller or limit his investigation. If Rosenstein refuses, as he certainly would, Trump could fire him and keep firing his successors until he found someone who would carry out his will.

The Justice Department has twice determined–during the Nixon and Clinton Administrations–that a President cannot be indicted or prosecuted while in office. It reached this conclusion based on the Constitution’s impeachment remedy and the special demands on the presidency. Mueller is bound by this and cannot indict Trump.

In the wrong hands, the prodigious powers of the presidency become worrisome. “[I]n every political institution, a power to advance the public happiness involves a discretion which may be misapplied and abused,” said James Madison. Trump has been threatening misapplication and abuse.

But another important constitutional principle is that hard presidential power doesn’t necessarily translate into effective presidential power. Trump should have learned this lesson when he fired former FBI director Jim Comey last year. The dismissal sparked a political firestorm and led Trump’s own nominee, Rosenstein, to appoint Mueller, who ramped up the investigation.

This political dynamic is the ultimate check on the presidency. It is why Trump–who has rained down ferocious tweet-threats toward the Justice Department–has not yet wielded his pardon pen or fired anyone else related to the Russia matter.

Trump’s threats without follow-through are a sign of presidential weakness. In the face of his howling disapproval, Mueller has been quietly building a comprehensive picture of any relationship among Trump, his associates and Russia during the 2016 election as well as any evidence of other crimes, including obstruction of justice.

Now That Mueller has had a year to gather this information, it is practically impossible for Trump to kill the investigation. In light of Mueller indicting 19 people and the many suggestive leaks, which are likely the tip of the iceberg, the political costs of firings or pardons are greater than ever.

Trump may nonetheless try to burn down the Justice Department. But even if he pardons everyone or finds someone willing to fire Mueller, which I doubt he can, the investigation will not end.

Mueller’s probe began as and at bottom remains a counterintelligence investigation designed to learn about and thwart Russian interference in U.S. elections. That task would continue even if there were no defendants to prosecute. Trump would be especially hard-pressed to shut it down, since so many people in his Administration have acknowledged the reality of the Russia threat.

More significant, any damning information Mueller has amassed is sure to come out–in a report, or by leaks or congressional subpoena.

This is the vital consideration. Mueller was never going to indict Trump. He is not allowed to, and if he tried, he would face a steep and lengthy uphill battle in court. His main goal was always to find out what happened and disclose it publicly so that Congress, by follow-up investigation and possibly impeachment, and the American people, through pressure on Congress and elections, can, if the evidence warrants, decide Trump’s fate.

This might seem like a weak and uncertain reed on which to ensure presidential accountability. But at the end of the day, it is the foundation of all constitutional constraints on the presidency.

This appears in the June 18, 2018 issue of TIME.

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