During his career, James Comey has served as a federal prosecutor, senior Justice Department official, corporate lawyer, and FBI director. With the release of his memoir on Tuesday, he is about to become a bestselling author.

But Mr. Comey’s most significant role may be yet to come.

If special counsel Robert Mueller’s investigation proceeds as many Trump critics are hoping, Comey could eventually emerge as the star witness at a congressional trial to possibly impeach President Trump for alleged obstruction of justice.

That goes a long way to explain why Comey is under such vicious attack on Twitter and elsewhere by the president and his supporters.

“Slippery James Comey… will go down as the WORST FBI director in history, by far,” Mr. Trump tweeted on Sunday.

In his book, “A Higher Loyalty: Truth, Lies, and Leadership,” Comey calls the president “unethical and untethered to the truth” and compares Trump to a mob boss obsessed with personal loyalty.

In an ABC News interview on Sunday night, Comey went further, saying he believes it is at least possible the president has been compromised by the Russians.

Significantly, he also said he thinks Trump may have engaged in obstruction of justice when he asked Comey to drop the FBI’s investigation of former National Security Adviser Michael Flynn.

“It’s certainly some evidence of obstruction of justice,” Comey told ABC’s George Stephanopoulos.

Ironically, legal experts say Comey’s book and associated national media blitz could wind up harming any potential impeachment case against the president, if, for example, discrepancies are found in the former FBI director’s recounting of events. And Comey’s broader attacks on Trump’s character could undermine his credibility as an impartial law enforcer.

“When you have witnesses going out into public and telling their stories, all kinds of things can happen,” says Bill Yeomans, a legal scholar at the Washington-based advocacy group, Alliance for Justice. “Comey has every right to write his book…. But the timing is terrible.”

But an obstruction of justice case against the president was never likely to be straightforward, anyway – for the simple reason that it is still a matter of significant debate among legal scholars as to how much authority the Constitution grants presidents when it comes to overseeing law enforcement and prosecutions.

Often a key to impeachment trials

Obstruction of justice is no small matter. It formed the core of impeachment efforts against former Presidents Richard Nixon and Bill Clinton.

At the heart of Trump’s alleged obstruction is a private meeting in the Oval Office between the president and then-FBI director Comey.

It was Feb. 14, 2017. Trump was three weeks into his presidency, and already Mr. Flynn, his national security adviser, had been forced out over allegations that he lied about a conversation with the Russian ambassador.

Flynn hadn’t just lost his job. He was now also under investigation by the FBI.

In his ABC interview, Comey said he suspected something improper was about to happen when Trump asked Vice President Mike Pence and Attorney General Jeff Sessions to leave the room to allow Trump and Comey to speak privately.

Once Trump and the FBI director were alone, the president made an unusual request. “[Flynn's] a good guy, the president said, according to Comey. “I hope you can let it go.”

Comey says he was so alarmed by the exchange that afterward he drafted a memo to himself to create a paper trail. “I am recording it… because the conversation will likely come back some day and [the president] may well lie about it,” Comey said in the ABC News interview. “I need to remember exactly what was said there. It could be evidence of a crime.”

Trump critics say that in addition to the Flynn request, the president has undertaken a series of questionable actions that create what they see as a mosaic of presidential obstruction. They cite a request by Trump that the national intelligence director and CIA director intervene with Comey to convince him to stop the Flynn investigation. (There is no indication they acted on this request.)

They note that Trump continued to press Comey, asking him to help lift a “cloud” over his administration. And that after Comey took no action, the president fired him on May 9, 2017.

In a meeting with the Russian foreign minister and a subsequent television interview, they say, Trump mentioned the Russia investigation as playing a role in his decision to fire Comey.

The president is also reported to have come close to firing Mueller in June and December 2017 but eventually backed down, according to The New York Times.

More recently, news outlets have reported that Trump is considering firing Deputy Attorney General Rod Rosenstein. It was Mr. Rosenstein who appointed Mueller as special counsel, and Rosenstein who has authorized Mueller’s widening investigative authority.

Trump maintains the Trump-Russia investigation is a politically motivated “witch hunt” that is undercutting his authority as president and the effectiveness of his administration.

An unresolved legal question

But behind the looming showdown between the president and the special counsel lurks a surprisingly unresolved legal question: What exactly constitutes obstruction of justice by a president?

It is a source of debate within the legal community, and there are a wide range of opinions. But it has never been fully litigated in the courts.

At issue isn’t whether a president can be held accountable for obstructing justice. The short answer to that question is that he can. The accepted method to do so is impeachment by Congress.

What remains unclear is whether a president can be said to be obstructing justice if the actions in question are in accord with powers granted to the president under the Constitution.

This is where many legal analysts diverge.

Some believe that any action by a president that undercuts an investigation, if undertaken with a corrupt motive to undermine that investigation, would constitute obstruction of justice. If that wasn’t so, they argue, the president would be holding himself above the law and would violate his oath to faithfully execute the laws.

“It is a difficult issue because we are dealing with a president exercising his presidential authority,” says Mr. Yeomans of Alliance for Justice. “But I am thoroughly convinced that there is enough evidence even in the public domain to make out obstruction of justice.”

Trump’s actions are more blatant than those of former President Richard Nixon, who resigned amid the Watergate scandal, says Yeomans, a former Justice Department official and former law professor.

“We have Trump saying over and over in public that what he is trying to do is impede the investigation,” he says. “I don’t think there is any lack of evidence.”

Others embrace a broader concept of presidential authority. The Constitution authorizes the president, as the nation’s sole chief executive, to do certain things – such as wield a veto, grant a pardon, appoint certain officials, and fire certain officials.

Under this view, Trump’s firing of Comey – or potential firing of Mueller or Rosenstein – would not constitute an abuse of power or obstruction of justice, however ill-advised it might be from a political perspective.

Trump’s actions in the Flynn episode are not inconsistent with actions routinely taken by presidents during the founding generation, says Saikrishna Prakash, a constitutional scholar and law professor at the University of Virginia School of Law. “Early presidents intervened in prosecutions all the time,” he says. “Washington, Adams, and Jefferson basically wrote letters to prosecutors telling them to start and stop cases.”

“Their view was that we are the chief executive. We decide when the laws ought to be enforced and when they ought not to,” Professor Prakash says.

Modern presidents generally don’t exert such hands-on control over day-to-day law enforcement. But under this view, as the nation’s sole chief executive, they could.

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In fact, Prakash argues the president’s authority extends well beyond the power to direct prosecutions and fire officials. He says it might even enable a president to issue a pardon for himself.

Yet even if Trump were to pardon himself, it wouldn’t mean necessarily that he’d escape accountability. “The pardon power doesn’t extend to impeachment,” Prakash notes.