The indictment of former Trump campaign chair Paul Manafort sets the stage for a historic test of the American legal and political systems, which could soon come under new attack from a president who has previously asked his advisers about firing Special Counsel Robert Mueller and pardoning those caught up in the Russia probe.

Mueller’s painstakingly detailed 31-page indictment of Manafort and former Trump campaign aide Rick Gates alleges that the two men concealed their ties to the government of Ukraine, kept huge amounts of money in offshore shelters, and laundered more than $18 million since 2006.

The indictment doesn’t allege that they colluded with Moscow to help Trump win the 2016 election, which the president and his aides are already using to try to undermine Mueller and dismiss the ongoing probes into team Trump’s ties to Russia as partisan witch hunts.

But the key thing to understand about the indictments — and the guilty plea of a lesser-known Trump adviser named George Papadopoulos — isn’t about their possible impact on the current course of the Trump-Russia scandal. It’s about their probable impact on what’s still to come.

Trump may decide to try to protect himself by firing Mueller or pardoning Manafort

That’s because Mueller’s moves increase the likelihood that campaign advisers or administration staffers who find themselves in the special counsel’s crosshairs may look to strike plea bargains where they swap damaging information on Trump in exchange for lesser charges. Papadopoulos is already cooperating with the Mueller team.

As a result, Trump may decide that he has no choice but to try to protect himself by firing Mueller or issuing preemptive pardons to Manafort or others ensnared in the investigation.

Either move would trigger a political and legal crisis on a scale that hasn’t been seen since Watergate, with federal courts having to decide whether to overturn any of the Trump pardons and Republicans facing a moment of truth about their willingness to substantively stand up to Trump instead of merely criticizing him publicly.

The investigation into Trump’s ties to Russia has been simmering along for months, and it wasn’t clear if, or when, it would move from a political scandal to a legal one. With Monday’s moves by the Mueller team, it has. The question is how far Trump is willing to go to protect himself from an investigation that threatens the future of his presidency — and whether Congress and the courts are up to the challenge.

This is a historic and dangerous moment for the Trump-Russia scandal

To understand where the scandal may be going, it’s important to understand the current state of play after Monday’s blizzard of legal moves by the Mueller team.

The day began with the release of Mueller’s indictments of Manafort and Gates, which detailed what prosecutors describe as a years-long scheme to profit from lobbying work on behalf of a pro-Russia Ukrainian political party while hiding the true source of the money. Manafort and Gates received a stunning $75 million between 2006 and 2016, much of it allegedly laundered through offshore bank accounts and shell companies. (The two men pleaded not guilty Monday afternoon.)

The indictment of Manafort had been widely expected for weeks, but what came next was a surprise. The Mueller team unsealed a guilty plea from a lesser-known Trump campaign adviser named George Papadopoulos, who admitted to lying to the FBI about his conversations with Joseph Mifsud, a professor with close ties to the Russian government who told Papadopoulos that the Russians had “dirt” on Hillary Clinton, including “thousands of emails.”

That disclosure is potentially an even bigger development than the Manafort indictment because it confirms that at least one Trump campaign adviser knew of Kremlin efforts to help Trump win the White House and was open to accepting that assistance. Whether Papadopoulos shared that information with others within the Trump campaign (or with Trump himself) remains an open question.

The Papadopoulos filings are written in the dry language of a legal document but include several sentences that should raise alarms throughout the Trump administration. In one of their court filings, Mueller’s team said the former adviser had been arrested in late July and has been secretly working with the prosecutors ever since. Papadopoulos “has indicated that he is willing to cooperate with the government in its ongoing investigation into Russian efforts to interfere in the 2016 presidential election,” Mueller’s team wrote in a separate court filing.

That raises questions about what kinds of information Papadopoulos has already provided to Mueller; whether he wore a wire or otherwise tried to help Mueller’s team gather evidence on other members of the Trump campaign or administration; and whether other members of team Trump are also quietly working with Mueller.

Former Justice Department spokesperson Matthew Miller pointed out on Twitter that the only person who knows the answers to those questions is Mueller himself.

Significantly, the plea agreement doesn’t outline who Papadopolous shared that offer with & how they reacted. But Mueller clearly knows. https://t.co/XMNgF3jY9S — Matthew Miller (@matthewamiller) October 30, 2017

And that’s why Trump may decide that the time has come to oust Mueller — or pardon people like Manafort and Papadopoulos — regardless of the possible consequences.

Congress and the courts may be the ones who decide what comes next

In the short term, the ball is clearly in Mueller’s court, since it’s his team that will be looking to flip more people like they did with Papadopoulos and to prosecute those who don’t cut a deal.

But that could very literally change at any moment depending on what Trump does next, with federal judges and congressional Republicans being the final arbiters of whether the president’s actions finally go too far.

Take the prospect of a Trump pardon of Manafort or others, like presidential son-in-law Jared Kushner, who are indicted down the road.

Trump has argued that “all agree the U.S. President has the complete power to pardon,” but that’s a vast oversimplification. As former Deputy Attorney General Harry Litman and former federal prosecutor Mark Greenberg argue at the Los Angeles Times, Mueller could go to court to argue that a Manafort pardon was invalid because Trump issued it to improperly block a criminal investigation and obstruct justice.

They write:

Indeed, if the pardon power were not so constrained, the president would in effect be above the law — an untenable position in our democracy. To take just one example, at the end of his term, he could hire a contract killer to murder a personal enemy and then pardon the hitman and himself on his last day.

Any challenge to Trump’s pardon would almost certainly end up at the Supreme Court. “Mueller v. Manafort would instantly become a case for the ages,” they write.

The other potential battleground would be in Congress, where a handful of Republican lawmakers have introduced legislation designed to protect Mueller from being fired by Trump.

A bill co-sponsored by Senate Judiciary Committee members Sens. Thom Tillis (R-NC) and Chris Coons (D-DE), would allow Mueller to challenge his removal in federal court if Trump ousted him. The other bill, co-sponsored by Sens. Cory Booker (D-NJ) and Lindsey Graham (R-SC), would allow judges to potentially prevent the firing in the first place.

In the aftermath of the Manafort indictment, Senate Minority Leader Chuck Schumer (D-NY) said Trump “must not, under any circumstances, interfere with the special counsel’s work in any way” and called on lawmakers from both parties to “ensure the investigation continues” even if Mueller is fired. Democratic Sen. Mark Warner of Virginia, the ranking member of the Senate Intelligence Committee, and Sen. Dianne Feinstein of California, the top Democrat on the Senate Judiciary Committee, made similar statements.

The problem is that neither of the bipartisan bills designed to shield Mueller have gone anywhere, and there are no signs yet that the Manafort indictment will change that.

Senate Judiciary Committee Chair Chuck Grassley (R-IA) said Monday that “the president should let the special counsel do his job," but leading Republicans have otherwise largely avoided the issue. House Speaker Paul Ryan, asked about the indictment by a Wisconsin radio station, switched the topic to tax reform. "Nothing is going to derail what we're doing in Congress,” he said.

The first chaotic months of the Trump presidency have been marked by a recurring pattern in which Republicans either try to ignore brazen Trump moves like his firing of FBI Director James Comey or offer only the mildest of rebukes.

As Yascha Mounk notes at Slate:

What’s strange about this is that we should, at this point, know what comes next. We have, after all, seen more or less the same pattern play out over and over since Trump took office. The White House intimates that it might destroy a key democratic norm that had once seemed unassailable. Pundits on TV confidently opine that Trump would never do something quite as crazy as this. Political scientists predict that he could never get away with crossing such a bright red line in such a brazen manner. ... But, inevitably, Trump does do the crazy thing.

Firing Mueller or pardoning Manafort would break with decades, if not centuries, of precedent for how American presidents treat the criminal justice system. It would also be the biggest test of how far Congress would be willing to go to rein in a rogue president since the darkest days of the Nixon presidency.

The US political and legal systems did their jobs during Watergate. It’s profoundly depressing that we have to ask whether they’ll do their jobs during the Trump presidency. It’s even more depressing to know that they might not.