Still, independent prosecutions have a history of ending with pardons, and Manafort has yet to hint that he’ll sing. The prospect of President Trump giving his former manager a free pass has sparked two fierce debates. Would a pardon that blocks the special counsel be constitutional? And if the special counsel were sidelined, could a state prosecutor take his place?

President Trump has tweeted that ‘all agree the US President has the complete power to pardon’. Indeed, Alan Dershowitz, Harvard Professor of Law, Emeritus, has stressed that the Constitution places no express limit on the President’s pardon power. But UCLA Law Professor Mark Greenberg believes, based on both English and American precedents, that the pardon power must be finite. After all, he argues, any power must be construed in light of other constitutional clauses like due process, and basic principles like the rule of law. Otherwise the President could lawfully invoke his war power to murder a personal enemy. By the same token, Greenberg and his colleague Harry Litman doubt that the President can issue a pardon for an improper purpose like thwarting an investigation.

Of course, President Trump has yet to pardon anyone in the special counsel’s sights. That may reflect the fear of a political backlash. Or it might reflect a subtle legal calculus. If Manafort is pardoned now, many ex-prosecutors note, then there’s no way he could incriminate himself on the pardoned crimes, and he might lose his constitutional right not to testify. Paradoxically, an up-front pardon might only loosen a witness’s lips.

That doesn’t remove the possibility of a later pardon. And it doesn’t remove the possibility that President Trump will simply sack the special counsel. Greenberg and Litman throw caution to the wind and outright predict Robert Mueller’s firing. ‘I think it's extremely likely,’ says Greenberg. ‘The investigation will continue to put huge pressure on the President, as more people are indicted and more people cooperate… Given what we know about the President and his tweeting habits, all there needs to be is one moment when he can’t resist temptation. There only has to be one morning at 5am. And once he tweets that he’s firing Mueller, we know that he always doubles down, and there’s no turning back.’

Let’s suppose the Mueller investigation is stymied, either by a termination or a pardon. Can state attorneys general step into the void? Fordham University Law Professor Jed Shugerman says absolutely; a second case could easily be brought under state law analogous to federal charges. Mueller has reportedly coordinated his investigation with the New York Attorney General. It’s true that New York has a double jeopardy statute that prosecutors would need to work around. Nevertheless, based on some of the charges Mueller appears to have deliberately avoided, he has some commentators convinced that state AGs will be an effective backstop.

Greenberg is receptive to the notion that state AGs may play an important role. The threat of convicting a peripheral figure for ordinary crimes sometimes leads to the exposure of extraordinary crimes by more central figures. That’s how US legal investigations are designed to work. And judges might be more open to creative interpretation of state law if the checking function of federal law is stymied.

University of Missouri Law Professor John Bowman cautions that state AGs can’t fully replace Mueller. Potential defendants who hold or held federal office, like Flynn, may argue they should be immune to state suit under the supremacy clause, which subordinates state to federal law precisely in order to protect the federal government from harassment by politicised local prosecutors. We’re ‘underestimating the practical legal barriers that arise the minute a state attorney general goes after essentially political offences,’ says Bowman. Moreover, the potential charges at the core of the investigation – interference with a federal campaign, or obstruction of a federal investigation – are inherently federal in nature. ‘Efforts to investigate that sort of conduct under state law can be fairly easily blocked or delayed,’ says Bowman.