House Intelligence Committee ranking Democrat Adam Schiff stands next to a photo of President Donald Trump in the Oval Office shaking hands with Russian Foreign Minister Sergey Lavrov. Photo: Bill Clark/CQ-Roll Call,Inc.

Last week saw a great deal of speculation about whether President Trump might either be impeached or prosecuted. The latter possibility came more into focus on Wednesday with the appointment of Robert Mueller, an FBI Director under presidents Bush and Obama, as a special counsel to lead the investigation into Russian involvement in the 2016 presidential election. Mueller will operate outside of the regular Justice Department hierarchy, allowing him to take the investigation wherever it leads.

The question of whether Trump has committed a crime while in office was amplified after it was revealed that he asked then–FBI director James Comey to back off the investigation of former national security adviser Michael Flynn’s ties to Russia. Comey did not back off, and Trump fired him two weeks ago.

To the extent that Trump himself is alleged to be entangled with Russia, his dealings with Comey (including a request for a loyalty pledge) all raise the possibility that Trump was acting to protect his own hide — and might have committed obstruction of justice.

Whatever the interaction was between the Russians, Flynn, Comey, and Trump, Mueller’s team will almost certainly keep digging until they are satisfied that they know what happened. If President Trump committed crimes, they will say so.

But what then? The criminal liability of presidents is a difficult and unsettled area of constitutional law. There are a lot of special considerations that come into play. The upshot of these considerations is that if a president has committed crimes in office, impeachment should come first. If you want to prosecute a president, you should probably wait until he is no longer president. Here is why:

1. The president might have immunity from criminal prosecution, at least while in office. The Supreme Court has never addressed presidential criminal immunity, and there are arguments on both sides, so nobody has a definitive answer on this question. But there is a good argument that acting presidents do indeed have legal immunity. The Justice Department has carried on for decades with the understanding that presidents are immune. They have never brought a criminal case against a president that would have tested the theory, so the courts have never had an opportunity to render a decision.

Why might the president be immune? He is nationally elected, and constitutionally occupies a sacrosanct office. Vast swathes of the Constitution and federal law are devoted to ensuring that the president — the head of state, the head of government, and the one-man embodiment of the executive branch — is always on duty and has immediate understudies waiting in the wings. As a result, only a properly accountable agent should have the power to interfere with his ability to fulfill his duties — as an arrest, a prosecution, and a conviction would do. Congress, through the impeachment power, is a more obviously proper agent than an unaccountable prosecutor would be.

2. Special prosecutors are constitutionally awkward, especially when they pursue the president. Because all of the prosecutors in the federal government are subordinate to the president, it makes sense to appoint special prosecutors when the president himself is under investigation. But nowhere does the Constitution require the use of special prosecutors, and the best reading of the Supreme Court’s case law on the matter suggests that it is unconstitutional for a special prosecutor to compromise the president’s ultimate control over the executive branch.

The most serious limit on Mueller or any special prosecutor is the fact that he can be dismissed. Constitutionally he must be removable by Deputy Attorney General Rod Rosenstein, who appointed him. The political reality is that, short of egregious misconduct, Rosenstein (and, more to the point, President Trump) cannot get rid of Mueller or end his investigation without paying a tremendous political cost. But the fact that the president has that ultimate power means that even a special prosecutor is in a poor position to prosecute him.

3. It’s also constitutionally awkward for a state prosecutor to prosecute a sitting president. Prosecuting a president in state court instead of federal court would avoid the issues described above. But it would create a new source of constitutional awkwardness: federalism. Giving one local official the power to arrest and prosecute the president is giving that local official the power to detain and frustrate the function of the federal government. If you don’t see the constitutional problem with one blue-state prosecutor, accountable only to the voters in her county, prosecuting President Trump for something she thinks is a crime, ask yourself how acceptable it would have been for a red-state prosecutor, accountable only to the voters in her county, to have prosecuted President Obama for something she thought was a crime.

4. The president is held in check by public opinion. One reason that the Constitution might give presidents the problematic powers and protections discussed above is that the Constitution also provides checks and balances. One check is that the president is politically accountable in a way that nobody else in the government is, let alone anybody in the private citizenry. If the president wants to resist or even fire a special prosecutor who is pursuing him, the courts might allow it — but the people might not. Indeed, if the president’s move is politically unpopular enough he might be forced to back down. President Nixon was able to get away with having Archibald Cox, the first Watergate special prosecutor, fired. But the political price he paid for that was so high that he was politically unable to fire Cox’s successor, Leon Jaworski. Jaworski eventually brought Nixon down because …

5. The president is definitely, unquestionably subject to impeachment. Gerald Ford famously said that an impeachable offense is whatever a majority of the House of Representatives says it is. A president could commit a crime that the House or Senate do not think is worth removing him for. Getting a House majority to take this momentous step is no easy matter. But the flip side is true as well: If the investigations against a president reveal facts damning enough to make him a political pariah, then nothing will stop Congress from impeaching and removing him — not even lawyerly arguments about the proper definition of obstruction of justice. You or I might be able to get acquitted in court on a legal technicality, and an ex-president might be, too, but the House and Senate are bound principally by politics, not by the legal niceties.

The Framers of the Constitution put Congress in charge of these decisions because allegations of criminality by a president will always be deeply and inextricably entangled in politics. Rather than allow court proceedings to be distorted by such considerations, the Constitution puts politicians in charge of deciding what is to be done with the president. If those politicians are not aggressive enough to satisfy their constituents, or if they are too aggressive, they will be accountable to those voters in short order.

6. A smart prosecutor will see the advantages of waiting. Once the president is no longer in office — a day that can be brought nearer through impeachment — his argument for immunity from prosecution becomes much weaker. This gives any prosecutor a good reason to be patient and wait, either for impeachment and removal, for resignation, or for the natural end of the president’s term. This principle was seen in action when President Clinton’s pursuer Robert Ray — Kenneth Starr’s successor — appeared ready to prosecute before he and Clinton negotiated a settlement on Clinton’s last full day in office.

7. Even if convicted, a sitting president might be able to pardon himself. A final reason that prosecuting a sitting president would be awkward is that, at least for federal charges, he could immediately grant himself a pardon. No one is sure whether a presidential self-pardon would be valid and there are good arguments on both sides. President Nixon considered pardoning himself, but decided against it. A president who tried to pardon himself would surely pay a huge political price. But any president who was politically weak enough that he was facing prosecution and impeachment (not necessarily in that order) would likely have nothing to lose. Of course, a president could try to pardon himself regardless of whether there was an active criminal investigation against him. But doing so could hasten his impeachment — which the pardon power cannot touch. Once again, impeachment saves the day.

When it comes to the presidency, politics often trump law. This reflects our constitutional design. In many ways, it is a feature, not a defect. But the upshot of it is that those interested in getting to the bottom of whether President Trump obstructed justice should look to Congress first and foremost.

Brian Kalt is a professor of law at Michigan State and the author of Constitutional Cliffhangers: A Legal Guide for Presidents and Their Enemies.