What does the ruling say?
Legal opinions on two occasions from the OLC concluded that criminally prosecuting a sitting president would undermine his or her ability to perform the duties of the executive branch.
"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 1973 guidance, written when President Richard Nixon was facing obstruction of justice charges.
“The spectacle of an indicted president still trying to serve as Chief Executive boggles the imagination,” the memo stated.
The other opinion – written in 2000 when charges of obstruction of justice were being leveled at former President Bill Clinton – read: "Our view remains that a sitting President is constitutionally immune from indictment and criminal prosecution."
What about impeachment? Can a sitting president face obstruction charges during an impeachment?
Yes. Both presidents who have faced impeachment – Nixon and Clinton – were accused of obstruction of justice.
But remember, impeachment is a political process. The House of Representatives, the body that would bring impeachment charges against a president is in the hands of the Democrats, but the Senate, the body that would conduct the “trial” for impeachment, has a Republican majority.
The Constitution said that a president can be removed from office if he has committed “treason, bribery, or other high crimes and misdemeanors.”
The standard of what constitutes “high crimes and misdemeanors” is vague, and can be defined by the Congress in an impeachment proceeding.
Did Mueller have to follow the guidance and not indict Trump if he felt he had proof of obstruction?
There is some room in the policy that would have allowed Mueller to bring charges in "extraordinary circumstances" as long as the U.S. attorney general gives his approval. Some attorneys say that a special counsel, such as Mueller, is not bound by the opinion since his work is not the usual work product of the Department of Justice.
Ken Starr, who was the independent counsel who investigated Clinton, said he did not believe he was constrained by the OLC guidance. However, Starr did not indict Clinton for obstruction.
Can a president be indicted once he has left office?
Yes, as long as the statute of limitations or the time prosecutors have to file charges after a crime has been uncovered, has not expired. Many federal crimes have a five-year statute of limitations which means if Trump is re-elected in 2020, he may be beyond the statute of limitations before he is out of office.
Special counsel Robert Muller speaks at the Department of Justice Wednesday, May 29, 2019, in Washington, about the Russia investigation.