WASHINGTON (Reuters) - The U.S. Constitution explains how a president can be removed from office for “high crimes and misdemeanors” by Congress using the impeachment process. But the Constitution is silent on whether a president can face criminal prosecution in court, and the U.S. Supreme Court has not directly addressed the question.

FILE PHOTO: U.S. President Donald Trump speaks during campaign rally at Mohegan Sun Arena in Wilkes-Barre, Pennsylvania, U.S., August 2, 2018. REUTERS/Leah Millis/File Photo

The question looms large with Special Counsel Robert Mueller preparing a report on his investigation into Russia’s role in the 2016 U.S. election, whether President Donald Trump’s campaign conspired with Moscow and whether Trump unlawfully sought to obstruct the probe.

The U.S. Justice Department has a decades-old policy that a sitting president cannot be indicted, indicating that criminal charges against Trump would be unlikely, according to legal experts.

Here is an explanation of the rationale behind the Justice Department policy and whether it applies to Mueller.

WHAT IS THE JUSTICE DEPARTMENT POLICY?

In 1973, in the midst of the Watergate scandal engulfing President Richard Nixon, the Justice Department’s Office of Legal Counsel adopted in an internal memo the position that a sitting president cannot be indicted. Nixon resigned in 1974, with the House of Representatives moving toward impeaching him.

“The spectacle of an indicted president still trying to serve as Chief Executive boggles the imagination,” the memo stated.

The department reaffirmed the policy in a 2000 memo, saying court decisions in the intervening years had not changed its conclusion that a sitting president is “constitutionally immune” from indictment and criminal prosecution. It concluded that criminal charges against a president would “violate the constitutional separation of powers” delineating the authority of the executive, legislative and judicial branches of the U.S. government.

“The indictment or criminal prosecution of a sitting President would unconstitutionally undermine the capacity of the executive branch to perform its constitutionally assigned functions,” the memo stated.

The 1973 and 2000 memos are binding on Justice Department employees, including Mueller, according to many legal experts. Mueller was appointed in May 2017 by the department’s No. 2 official Rod Rosenstein.

But some lawyers have argued that the nation’s founders could have included a provision in the Constitution shielding the president from prosecution, but did not do so, suggesting an indictment would be permissible. According to this view, immunity for the president violates the fundamental principle that nobody is above the law.

Nixon himself in 1977 offered an opposite view when he told interviewer David Frost, “Well, when the president does it that means that it is not illegal.”

COULD MUELLER INDICT TRUMP DESPITE THE EXISTING POLICY?

Possibly. The Justice Department regulations governing Mueller’s appointment allow him to deviate from department policy in “extraordinary circumstances” with the approval of the U.S. attorney general, the nation’s top law enforcement official. Trump appointee William Barr currently holds that post.

Some legal experts have suggested Mueller could invoke this exception if he has uncovered serious wrongdoing and lacked confidence in the ability of the divided Congress to hold Trump accountable. Some lawyers also have said Mueller is not bound by the 1973 and 2000 memos because he is not a typical employee of the department.

Ken Starr, who investigated President Bill Clinton in the 1990s in the somewhat different role of independent counsel, in 1998 conducted his own analysis of the question of whether a sitting president can be indicted, indicating he did not consider the 1973 Justice Department memo binding on him.

Starr did not indict Clinton in his investigation involving the president’s relationship with a White House intern named Monica Lewinsky, but lawyers in his office concluded he had the authority to do so, according to a once-secret internal memo made public by the New York Times in 2017.

After the independent counsel statute under which Starr was named expired in 1999, the Justice Department devised procedures governing the appointment of special counsels to handle certain investigations. Mueller was named after Trump fired FBI Director James Comey, who had been overseeing the agency’s Russia probe.

COULD TRUMP BE REMOVED FROM OFFICE AND THEN PROSECUTED?

Yes. There is no debate over whether a former president can be indicted for conduct that occurred while in office. In fact, President Gerald Ford, who succeeded Nixon after his resignation, was mindful of this when he granted “a full, free, and absolute pardon unto Richard Nixon for all offenses against the United States which he, Richard Nixon, has committed or may have committed.”

The statute of limitations - restricting the time within which legal proceedings such as a prosecution may be brought - may work to Trump’s benefit if he is re-elected in 2020 and serves a full two four-year terms as president until January 2025.

Many federal crimes have a five-year statute of limitations, meaning prosecutors have five years from the date the conduct at issue occurred to bring an indictment. That means criminal charges against a re-elected Trump could be time-barred.

Some lawyers have said that, as a matter of fairness, the normal rules on timeliness of charges should not apply to the president. The issue potentially could be resolved in the courts.