Special counsel Robert Mueller is now investigating whether President Donald Trump has committed obstruction of justice, the Washington Post reports. This news should shock precisely no one: Mueller’s mandate specifically authorizes him to investigate obstruction of justice allegations arising out of his probe. But the report underscores the seriousness of the obstruction allegations against Trump — and the strength of the evidence already amassed.

In a nutshell, the case against President Trump consists of the following: The President intimated to then-FBI Director James Comey in February that Comey ought to shut down the bureau’s investigation of former Trump National Security Adviser Michael Flynn. When Comey rebuffed him, President Trump sought to enlist Director of National Intelligence Daniel Coats in an effort to stop the Flynn probe.

Making matters worse, President Trump then fired Comey and said publicly that the firing was related to the Russia investigation — a statement that might be interpreted as an implicit threat to Comey’s replacement that he should bring the probe to a halt if he wanted to keep his job. Whether or not any of these actions would amount to obstruction in isolation, they sum up to a course of conduct that might very well place President Trump on the wrong side of the criminal law.

Trump may be shielded from criminal indictment by virtue of his status as president, and he may remain shielded from impeachment by virtue of the fact that his own party controls both chambers of Congress. On the law, though, the argument that Trump is guilty of obstructing the FBI’s Flynn probe is quickly gathering steam.

What exactly is obstruction of justice?

While “obstruction of justice” has become something of a buzzword in recent weeks, it is an offense with a precise legal definition, criminalized by several federal statutes. The two that are most likely to apply in this case are section 1505 and 1512 (c) of Title 18 of the US Code. Both statutes impose liability on anyone who “corruptly” influences a proceeding or who “endeavors” or “attempts” to accomplish that result. The term “corruptly” means “acting with an improper purpose.” And importantly, the endeavor or attempt need not be successful. As Chief Justice William Rehnquist wrote in a 1995 opinion, “the endeavor must have the ‘natural and probable effect’ of interfering with the due administration of justice,” regardless of whether any interference or influence ultimately occurs.

Section 1505 applies only when the defendant attempts to influence a “pending proceeding” before a federal department or agency or a congressional committee. Lawyers and law professors disagree about whether an FBI investigation can count as a “pending proceeding” for purposes of section 1505. A federal district court in Louisville— in a 1981 case called United States v. Higgins — held that an FBI investigation is not the type of proceeding to which the statute applies, and a Justice Department manual takes the same position (citing the Higgins decision).

But as Helen Klein Murillo and Benjamin Wittes write in an excellent analysis of the issue on the Lawfare blog, the Higgins decision is highly questionable. The Higgins court held that the phrase “pending proceeding” in section 1505 includes only proceedings involving an agency “with rulemaking or adjudicative authority in addition to investigative functions” — even though that limitation is found nowhere in the text of the statute. Moreover, the Higgins decision is not binding precedent outside the Western District of Kentucky (or even inside it), since it was the product of a single district judge’s pen.

In any event, if that section of the law does not apply in this case, section 1512(c) still might, as former federal prosecutor Randall Eliason has noted. Section 1512(c), enacted after Enron as part of the Sarbanes-Oxley Act, can be used to charge anyone who “corruptly obstructs, influences, or impedes any official proceeding, or attempts to do so.” Such “official proceedings” include federal grand jury investigations. Several news organizations have reported that a federal grand jury in the Eastern District of Virginia has begun an investigation of Michael Flynn, the president’s former national security adviser, and his interactions with Russian officials — and that investigation might be enough to trigger the application of section 1512(c).

President Trump might have obstructed the investigation into Flynn even if he knew very little about it

Significantly, it doesn’t matter for the purposes of the law whether the grand jury probe was underway at the time of President Trump’s alleged obstruction: The statute clearly states that “an official proceeding need not be pending or about to be instituted at the time of the offense” — Trump’s obstruction could violate section 1512(c) if it interferes with a grand jury proceeding later in time. It also doesn’t matter whether President Trump knew about the proceeding: The question is instead whether the grand jury proceeding was foreseeable to Trump at the time of the offense.

So to sum up so far: The central question under either section 1505 or section 1512(c) is whether President Trump, acting with an improper purpose, took steps that had the natural and probable effect of influencing the FBI’s investigation into ties between the Russian government and Michael Flynn. If the answer to that question is “yes,” Trump still might have additional arrows in his quiver: He might argue that the FBI investigation is not the sort of proceeding to which section 1505 applies, or that the grand jury inquiry into Flynn wasn’t foreseeable at the time of the attempted obstruction.

But showing that Trump acted with an improper purpose and took steps that had the natural effect of influencing the FBI’s Flynn probe would go a long way toward establishing that he is guilty of obstruction.

To make a judgment about obstruction, look beyond the solo meetings with Comey and examine the full picture

The strongest single piece of evidence against President Trump is his Valentine’s Day tête-à-tête with the president in the Oval Office. According to Comey’s testimony, Trump cleared the room so that he would be alone with the FBI chief — which itself put Comey on alert that something unusual was happening. The president then said to Comey, “I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go.” That’s not an explicit order, but at least two federal courts of appeals have found that “I hope” statements, when spoken by a defendant with potential sway over the statement’s target, can constitute obstruction of justice.

Moreover, as one federal court has put it, “specific incidents which do not amount to obstruction of justice when viewed individually can constitute obstruction when viewed in their entirety.” And we have learned more this month about the entirety of the president’s conduct. First, the Washington Post has reported that the president had asked Director of National Intelligence Daniel Coats in March if Coats could press Comey to pull back the FBI’s investigation of Flynn. The president’s request, according to the Post, came in a private meeting with only Coats and CIA Director Mike Pompeo in the room.

And second, under questioning from Senate Intelligence Committee ranking member Mark Warner last week, both Coats and National Security Agency Director Mike Rogers refused to say publicly whether Trump had requested that they intervene in the FBI’s Russia probe. Coats and Rogers both said that they did not feel “pressured” to influence the FBI investigation, but neither man would deny that Trump had asked him to step in.

All this comes against the backdrop of what was already in plain sight: that Trump fired Comey on May 9 and then indicated in an interview with NBC News’s Lester Holt that the firing was connected to the FBI’s continued inquiry into the “Russia thing.” With the Comey firing and the Holt interview, the president sent a not-so-subtle message to Acting FBI Director Andrew McCabe that he might not keep his job for long unless he shut down the bureau’s Russia probe. (As it happened, Mueller took over the investigation last month. McCabe was a finalist for the FBI directorship, but Trump ultimately nominated Atlanta attorney Christopher Wray for the post.)

So whether or not the president’s request that Comey “let Flynn go” would be obstruction of justice in its own right, the appropriate question is whether that request — along with Trump’s requests to Coats and possibly the CIA and NSA chiefs as well, plus the firing of Comey and the subsequent remarks to Holt — all combine to constitute an endeavor that was reasonably likely to influence the direction of the FBI’s inquiry.

To be sure, it might not be improper for the president to tell the FBI director to back off an investigation if the president thought it was a misallocation of bureau resources. The president, after all, is the head of the executive branch, and he has a duty to make sure that federal law enforcement agencies are performing their functions efficiently. President Trump might argue that he asked Comey to back off the Flynn investigation because he knew Flynn was innocent and thought that the dead-end investigation of his former adviser was consuming too much of the FBI’s time and energy. (My colleague Eric Posner and I have set out this argument in more detail here and here.)

But this defense becomes less persuasive if indeed Trump asked intelligence officials to intervene in the FBI’s inquiry. If Trump’s request to Comey was motivated purely by FBI resource allocation concerns, then it is hard to understand why the president would have enlisted other intelligence chiefs in the effort. It is also hard to understand why the president would have cleared the Oval Office before asking Comey to back off the Flynn probe. Certainly, the circumstantial evidence suggests that Trump’s attempt to influence the FBI investigation was not motivated by a concern for preserving the bureau’s crime-fighting or counterterrorism capabilities.

Indictment — or impeachment?

In a May 10 memo to all federal prosecutors, Attorney General Jeff Sessions said that Justice Department attorneys “should charge and pursue the most serious, readily provable offense.” That instruction applies to special counsel Robert Mueller, who must comply with all Justice Department policies unless Deputy Attorney General Rod Rosenstein allows Mueller to deviate from department norms. (Rather ironically, Rosenstein’s decision not to give Mueller the same independence as the special prosecutor in the Bush-era CIA leak inquiry means that the Sessions memo’s aggressive charging policy applies with full force to the new special counsel.)

But of course, the president is not an ordinary criminal suspect. Even if President Trump did commit obstruction of justice, it is far from clear that a sitting president can be indicted. The Constitution does not explicitly bar the prosecution of a president, but the Justice Department’s Office of Legal Counsel has concluded that “a sitting President is constitutionally immune from indictment.” Mueller might well take the view that the Office of Legal Counsel’s conclusion trumps the Sessions memo on this point.

That still leaves the question of impeachment. Article II of the Constitution provides for impeachment and removal in the case of “Treason, Bribery, or other high Crimes and Misdemeanors.” And while then-Representative Gerald Ford famously quipped that “[a]n impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history,” that is not the same as saying that impeachment is a standardless exercise. (One could just as easily say that the Constitution means whatever five justices of the Supreme Court consider it to mean at a given moment.)

While the meaning of “high Crimes and Misdemeanors” is endlessly debated, there is broad bipartisan consensus that obstruction of justice qualifies. The House of Representatives voted 381-0 to impeach Texas Federal District Court Judge Samuel Kent for obstruction of justice in 2009. (Kent resigned before the Senate could convict.) Obstruction of justice was also one of the grounds on which President Clinton was impeached in 1998, as well as the first article of impeachment filed against President Nixon in 1974. Notably, the charge against Nixon was based on his efforts to interfere with the FBI’s investigation of the Watergate break-in — suggesting that even if obstruction of an FBI probe does not violate section 1505, it nonetheless amounts to a high crime or misdemeanor under Article II.

None of this is to suggest that indictment or impeachment is imminent. What this does suggest is that the Valentine’s Day request to Comey — combined with Trump’s reported request that Coats intervene in the Flynn investigation, Coats’s nondenial of that report, and his remarks to Lester Holt — form a plausible though not open-and-shut case of obstruction. Mueller’s decision to look into the question seems like a relatively easy one. Whether ultimately to recommend charges against Trump — either while he is in office or after he leaves the White House — will be a much more difficult judgment call.

Daniel Hemel is an assistant professor at the University of Chicago Law School.

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