Special counsel Robert Mueller appears to be building a case that President Donald Trump’s interference with the Russia probe constitutes obstruction of justice.

Just this Tuesday, we learned that Mueller’s team has interviewed former FBI Director James Comey and Attorney General Jeff Sessions, clear signs that the special counsel is looking into the circumstances surrounding Comey’s firing. We also learned that Mueller is attempting to question Trump himself about the Comey firing, as well as the firing of former National Security Adviser Michael Flynn — a request you’d expect from a prosecutor closing in on their target.

And here’s the thing: the case against Trump on obstruction grounds is very strong — stronger even than the evidence that the Trump campaign broke the law by colluding with Russian’s campaign to influence the election.

“If Trump exercises his power — even his lawful power — with a corrupt motive of interfering with an investigation, that’s obstruction,” says Lisa Kern Griffin, an expert on criminal law at Duke University. “The attempt is sufficient, and it seems to be a matter of public record already.”

If Mueller feels he has enough evidence, then he could seek permission to indict and prosecute Trump. It’s not clear that charges can actually be brought against a sitting president, but Mueller’s findings could nevertheless be turned over to Congress — and serve as the centerpiece of any impeachment proceedings against Trump.

The obstruction, it seems, is the biggest legal and political threat to President Trump to emerge from the Mueller probe so far.

There are basically two reasons Griffin and other legal observers believe Mueller has such a good case obstruction case. First, the evidence of obstruction is, from what we know publicly, far stronger than the evidence that Trump himself was involved in with Russian efforts to interfere in the 2016 election. Second, the crime of obstruction is legally straightforward, whereas it’s not obvious which laws Trump would have violated by accepting Russian assistance during the election.

The public, obviously, doesn’t know everything Mueller does. It could be that the collusion case is a lot clearer, or the obstruction case a lot murkier, than it appears from the outside.

But what we do know suggests that Mueller is taking the obstruction charge seriously, and that his chances of making his case are quite good — unless Trump decides to fire him or his boss.

The evidence that Trump obstructed justice seems stronger than the evidence of collusion

We know for a fact that Mueller is looking into potential obstruction charges.

On January 4, the New York Times’s Michael Schmidt reported that Trump ordered the White House counsel, Don McGahn, to convince Attorney General Jeff Sessions not to recuse himself from the Russia investigation. When McGahn failed, “the president erupted in anger in front of numerous White House officials, saying he needed his attorney general to protect him,” Schmidt writes. “Mr. Trump said he had expected his top law enforcement official to safeguard him.”

The incident suggests that the president wanted law enforcement officials to protect him from criminal charges. And indeed, Schmidt reports that the episode was initially discovered by Mueller “as he investigates whether Mr. Trump obstructed the F.B.I.’s Russia inquiry.”

The irony, according to many experts, is that this kind of effort to block the Russia investigation has likely created a bigger legal problem for Trump than anything he may have done with Russians.

“So-called ‘offenses against the administration of justice’ — like obstruction of justice or lying to the FBI — are often easier to prove than the underlying criminality that the subject was trying to hide,” says Jens David Ohlin, a professor of law at Cornell. “In Trump’s case, this is undoubtedly true.”

There’s clear evidence that some of Trump’s campaign staff were in contact with Russian officials, and even interested in receiving their help in the election. The best example is the June 2016 Trump Tower meeting, where Donald Trump Jr. met with a Kremlin-linked attorney who was promising to provide dirt from the Russian government on Hillary Clinton.

But to link Trump himself to collusion, you’d need to prove that he had personally approved of this meeting, or that the Trump campaign as a whole was involved in a broader plot to work with the Russian government. In the absence of this evidence, experts say it would be very hard to charge the president with any kind of collusion-related crime. And no such evidence exists, at least in the public record.

“Nothing, to my knowledge, has come out that implicated [Trump] directly,” says Asha Rangappa, a former FBI special agent and current associate dean of Yale University.

The public evidence that Trump committed obstruction of justice — defined as “corruptly” attempting to “influence, obstruct, or impede, the due administration of justice” — is quite a bit stronger, particularly when it comes to the Comey firing.

Technically, the FBI director serves at the pleasure of the president and can be fired at any time and for any reason. However, the general belief among experts is that it’s not legal for the president to use this power in an attempt to shield himself or his family members from criminal investigation. Crucially, it does not matter whether Trump successfully shielded himself from scrutiny — all that matters is that he tried to.

“The obstruction statutes are very broad,” Griffin explains. “It matters whether Trump ‘endeavors’ to impede the DOJ and congressional investigations at any turn.”

This all hinges on what was in the president’s mind when he fired Comey. If Trump did it because he thought Comey was doing a poor job supervising the Russia investigation, that’s perfectly legal. If he did it because he was worried that Comey’s investigation might end up uncovering some kind of misdeeds by the Trump family — whether Russia-related, financial, or otherwise — then that’s obstruction.

Typically, proving intent is difficult for prosecutors, because establishing what a person was thinking at a particular moment is inherently challenging. But Trump has made things easier for Mueller — by repeatedly, even publicly, admitting that he fired Comey out of hostility to the Russia investigation.

There’s an interview with NBC’s Lester Holt in which Trump admits to removing the FBI chief because he felt “this Russia thing with Trump and Russia is a made-up story.” There was Trump’s meeting with Russian Foreign Minister Sergei Lavrov the day after the firing, where he told the foreign diplomat that “I faced great pressure because of Russia [and now] that’s taken off.” And then there are his endless tweets about how he thinks the Russia investigation is a “witch hunt” and should be shut down.

“He’s kind of making the case himself for Mueller [on] obstruction,” Rangappa concludes.

Firing Comey isn’t the only possible Trump action that could land him in hot water on obstruction charges. But the firing is the clearest example based on what we know right now — and one that seems more likely to form the backbone of any charges against Trump than anything directly related to Russian collusion.

We are still a very long way away from indictment or impeachment

Even if there were evidence of Trump personally plotting with Russia — and it may well be, squirreled away in Mueller’s files — it’s not clear whether this would constitute a crime. “Collusion” is not a legal term; Mueller would have to figure out which specific law Trump’s actions violated, which according to legal experts could be trickier than it might seem.

By contrast, there’s no similar lack of clarity about obstruction of justice. The articles of impeachment against both Richard Nixon and Bill Clinton included allegations of obstruction.

Still, as strong as Mueller’s obstruction case appears to be, there are several major hurdles he’d have to clear before bringing a case.

Under the law governing special counsels, Mueller does not have the power to simply announce that he thinks the president has committed a crime and then indict him. He doesn’t even have authority to issue a report saying that the president committed crimes, a step well short of criminal prosecution.

To do either, he would need approval from the top Department of Justice official supervising the investigation. Typically, this would be the attorney general — but since Sessions recused himself from the Russia probe, it falls to his second in command, Deputy Attorney General Rod Rosenstein.

“[Mueller] really can’t just walk into a federal court and go through the normal criminal process,” Rangappa explains. “He brings his findings to Rosenstein, he says, ‘I want to bring these charges,’ or, ‘I’m declining to bring these charges,’ and [Rosenstein] can approve or not.”

This is a crucial point of vulnerability in the case against Trump. Legally, Trump would need Rosenstein’s approval to fire Mueller — but Trump could fire Rosenstein all on his own. You’re already seeing support for this course of action building; there’s a faction of Republicans in Congress who believe that Mueller’s probe is biased against Trump and that its leader needs to be fired.

The decision would then fall to the next person in the DOJ hierarchy, Associate Attorney General Rachel Brand, to fire Mueller. If she refused, Trump could fire her and try again with her successor. The president could also muzzle Mueller, and forestall an indictment, by firing Attorney General Jeff Sessions and appointing a friendly attorney general who wouldn’t have to recuse himself from supervising the Russia probe.

These are hard things to do. Firing Mueller might require firing a massive portion of the DOJ, and new attorneys general need congressional approval. But they are potential avenues through which Trump could use his powers to take control of the investigation.

Assuming Rosenstein remains in office and concludes that Mueller’s evidence against Trump is sufficient to support an obstruction case, the question becomes what to do about it.

Current DOJ policy is that sitting presidents cannot be indicted in criminal court as a matter of constitutional law. Some scholars dispute that; my colleague Dylan Matthews has an excellent overview of the debate. But the fact that an indictment is constitutionally dubious means that Rosenstein is unlikely to greenlight bringing charges.

An alternative would be Rosenstein issuing a report detailing evidence of Trump’s alleged crimes, a document lawmakers could use in any impeachment proceedings against Trump.

Impeachment is the clearest and most constitutionally straightforward punishment for crimes by a sitting president, and the one most likely to lead to Trump’s removal from office. This means that no matter how strong Mueller’s evidence is, Trump’s fate would most likely to be decided on Capitol Hill — in a profoundly political vote.

For more on this, listen to Worldly, Vox.com’s foreign policy podcast: Why Mueller’s obstruction of justice investigation should really scare Trump.