One of the most fundamental questions facing Special Counsel Robert Mueller in his investigation of President Trump is what to do if he uncovers evidence of wrongdoing by Mr. Trump.

Since 1973, the Justice Department’s Office of Legal Counsel has maintained a policy that a sitting president may not be prosecuted or indicted. That policy was first articulated during the Nixon administration’s Watergate scandal, and was reaffirmed in 2000 following the assorted scandals of the Clinton presidency.

The policy seeks to insulate the nation’s chief executive from prosecutorial pressures that would “impermissibly undermine the capacity of the executive branch to perform its constitutionally assigned functions,” according to the formal policy statement announced during the Clinton administration.

Now, 19 months into the Trump-Russia investigation, legal analysts are questioning whether Mr. Mueller should jettison the 45-year-old Justice Department policy, if necessary, to indict Trump.

Much of the recent debate is being driven by Trump critics who are fearful that if Mueller has no ability to prosecute the president, Trump will escape accountability. Others have held the same position on the thorny constitutional issue through decades of presidential scandals.

“I just think [the policy] is wrong for the reason that nobody is above the law,” says Paul Rosenzweig, a senior fellow at the R Street Institute, who served as a senior counsel during Independent Counsel Kenneth Starr’s investigation of President Bill Clinton.

Despite his legal assessment, Mr. Rosenzweig says he believes Mueller will follow the Justice Department’s policy and not seek to return an indictment charging Trump.

“This has been their consistent policy no matter who the president is,” he says. “It was first adopted for Nixon. They reaffirmed it for Clinton. And if they are called upon to do it, I’m sure they will reaffirm it for Trump.”

To be clear, Mueller has presented no evidence of wrongdoing by Trump – at least not yet. This week, The Washington Post reported that Mueller has told Trump’s lawyers that the president is not currently a criminal “target” of his investigation. That suggests Mueller’s investigators have found no substantial evidence of wrongdoing by the president.

But that could change.

Investigators are looking into whether Trump and members of his campaign conspired with Russia to meddle in the 2016 election and whether the president obstructed justice, in part by firing the FBI director in May of 2017.

Trump has called the ongoing special counsel investigation a “witch hunt” that unfairly questions the legitimacy of his presidency while undermining his effectiveness as the nation’s chief executive.

The debate over a possible presidential indictment is merely a hypothetical question at this point. But it is a question of constitutional magnitude with no settled answer, according to legal experts.

“The question in this case isn’t ‘Should the president be above the law?’ It is ‘What is the law?’ ” says Brian Kalt, a constitutional scholar and law professor at Michigan State University, and author of the book “Constitutional Cliffhangers: A Legal Guide for Presidents and Their Enemies.”

“If this case comes up to the Supreme Court, we are not deciding what we think of Trump or what we think about immunity,” he says. “We are deciding what we think the Constitution requires.”

Constitutionally indispensable?

Mueller was appointed special counsel under Justice Department regulations that require him to comply with the “rules, regulations, procedures, practices and policies” of the Justice Department.

That suggests Mueller is bound by the no-prosecution policy. But some analysts say such a restriction is inconsistent with the entire purpose of having a special counsel who can wield independent judgment in the face of suspected presidential wrongdoing.

There is nothing in the Constitution explicitly calling for presidential immunity from criminal prosecution, these analysts say.

“The Constitution creates an executive branch with the president under a sworn obligation to faithfully [execute] the law,” wrote law professor Ronald Rotunda in a 1998 memo to Mr. Starr during the investigation of the Clintons. “The Constitution does not create an absolute monarch above the law.”

Others counter that the structure of the Constitution implies that the president must be immune from prosecution while in office.

When the president takes office after winning a national election, he becomes the sole head of the executive branch of government. He does not share that authority with anyone else. As Supreme Court Justice Stephen Breyer once observed, the founders sought to establish a chief executive who is “constitutionally indispensable.”

“He is the only person in the government who has that kind of role, where taking him off the job is a big deal,” Professor Kalt says. “Taking the vice president off the job – not a big deal. Taking a Supreme Court justice off the job – not a big deal. Members of Congress, there are hundreds of them, and there are ready replacements.”

But the president is different. “You shouldn’t take the president off the job unless you have a legitimate way of doing it,” Kalt says. “The Constitution sets up that legitimate way by saying: If anyone is going to take him off the job, it will be Congress, through impeachment.”

Rosenzweig agrees there are practical arguments for why indicting and prosecuting a sitting president could hinder the government.

“You don’t want the most powerful man in the world to be spending 9 to 5 working on nuclear war with North Korea and then from 5 to 8 worrying about what his testimony is going to be,” he says.

But under this distraction theory, it could be argued that a president should not be subject to investigation at all, or to civil lawsuits for private conduct, or to have evidence of presidential wrongdoing presented to a grand jury. And each of these actions has been upheld by courts.

Mueller's options

In most criminal investigations in the US, evidence of wrongdoing is presented to a grand jury comprised of ordinary Americans who decide in secret proceedings whether the prosecutor’s evidence is strong enough to return an indictment.

If Mueller follows the Justice Department’s no-prosecution policy, instead of seeking an indictment he would present any gathered evidence implicating the president to Deputy Attorney General Rod Rosenstein, who appointed Mueller as special counsel and is overseeing the Trump-Russia investigation.

Mr. Rosenstein would then decide what to do with that evidence. He could keep much of it secret. He could make some or all of it public, or he could send it to Congress.

In that case, it would then be up to Congress to decide whether to begin impeachment proceedings.

But those aren’t the only options. There might be other avenues open to Mueller, with Rosenstein’s support. For example, he might name Trump as an unindicted co-conspirator within a broader indictment charging Trump aides, associates, and even family members.

Under this option, the president technically wouldn’t be prosecuted, but his alleged criminal actions would be exposed to public view and could form a basis for impeachment.

That’s what happened with President Richard Nixon during the Watergate investigation.

Despite the Justice Department’s 1973 memo concluding the president could not be indicted, Watergate Special Prosecutor Leon Jaworski presented evidence to a grand jury that subsequently named Mr. Nixon as an unindicted co-conspirator in the Watergate cover-up.

The grand jury also asked that a sealed briefcase containing evidence against Nixon be delivered to investigators who were already preparing for impeachment proceedings against Nixon in the House of Representatives.

The impeachment never took place. Instead, Nixon resigned as president. Any possibility of a subsequent prosecution of Nixon once he left office ended when he was pardoned by President Gerald Ford.

In the case of President Clinton, Independent Counsel Starr considered whether or not to present a criminal case against Mr. Clinton to a grand jury. A 56-page legal memo commissioned by Starr strongly argued that a president could be charged and prosecuted despite the Justice Department’s no-prosecution policy.

“It is proper, constitutional, and legal for a federal grand jury to indict a sitting president for serious criminal acts that are not part of, and are contrary to, the president’s official duties,” wrote Professor Rotunda, an adviser to the Independent Counsel, in the memo.

The confidential memo was obtained from the National Archives through a Freedom of Information Act request filed by The New York Times.

Despite the aggressive stance urged in the memo, Starr decided against indicting Clinton and instead sent a 445-page report to Congress. The Republican-controlled House authorized two articles of impeachment against Clinton for perjury and obstruction of justice. But the Senate declined to convict, splitting 50-50 on the obstruction charge and voting 45-55 on the perjury charge. It takes 67 votes to convict and remove a president from office.

Stigma and opprobrium

In his 2000 memorandum opinion upholding the no-prosecution policy during the Clinton administration, Randolph Moss, then the Assistant Attorney General for the Office of Legal Counsel, wrote that the burdens associated with a criminal prosecution of the president “would be so intrusive as to violate the separation of powers.”

“In times of peace and war, prosperity or economic crisis, and tranquility or unrest, the president plays an unparalleled role in the execution of the laws, the conduct of foreign relations, and the defense of the nation,” Moss wrote. “Indictment alone risks visiting upon the president the disabilities that stem from stigma and opprobrium associated with a criminal charge, undermining the president’s leadership and efficacy, both here and abroad.”

Moreover, he added, the Constitution identifies a specific mechanism for removing a president from office – and it does not involve a prosecutor. “The Framers considered who should possess the extraordinary power of deciding whether to initiate a proceeding that could remove the president…, and placed that responsibility in the elected officials of Congress.”

Rotunda’s memo urging the indictment and prosecution of Clinton presented a sharply different perspective on the issue. Many federal officials and judges are subject to removal by impeachment, he noted, but that provision does not confer upon them immunity from enforcement of federal criminal statutes. The president is no different, he argued.

“No legal precedent has ever concluded that the president is immune from the federal criminal laws,” wrote Rotunda, who passed away last month. “In fact, the cases have suggested the contrary.”

Yet although he presented an aggressive argument supporting the independent counsel’s authority to indict a sitting president, Rotunda wrote in a Washington Post essay last year that he did not believe that Mueller has the authority to indict Trump.

That’s because Starr was operating under a federal independent counsel statute, passed by Congress and signed by Clinton, while Mueller’s appointment is pursuant to Justice Department regulations, Rotunda said.

All about timing

Critics of the no-prosecution policy are afraid that it may allow a president to avoid accountability and punishment for wrongdoing.

Others note that the no-prosecution policy only applies while a president is in office. There is nothing preventing federal or state prosecutors from indicting and prosecuting a president once he completes his term or is removed from office through impeachment.

“This immunity is not about immunity as such, it is about timing,” Professor Kalt says. “They are not saying he can’t be prosecuted, they are saying he can’t be prosecuted now.”

Such a delay might prevent certain crimes from being held to account if the five-year statute of limitations expires while the president is still in office. But legal analysts stress that the no-prosecution policy is not a get-out-of-jail-free card.

“Congress can hasten the day that the president leaves office,” Kalt says. And voters may have something to say as well.

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“The bottom line is: If a president has committed a crime, even if you can’t prosecute him while he is in office, he is not going to be in office forever,” Kalt says.

“We do not have kings. Eventually, he will leave.”