When George W. Bush checked into the hospital for routine colonoscopies in 2002 and 2007, the procedures required him to be sedated, unable to act in an emergency. So he invoked a provision of the Constitution that temporarily put his vice president in charge.
That same provision of the 25th Amendment can be invoked whenever a president is, for any reason, unable to carry out the duties of the office. It was adopted after the assassination of President John F. Kennedy, because Congress realized there was no clear answer to who was supposed to be in charge.
It addressed the Kennedy situation by saying that whenever a president dies or resigns, the vice president becomes president.
But a different provision covers presidential illnesses like Covid-19.
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A president who knows he is or will be unable to perform the duties of the office notifies Congress, and the vice president becomes acting president. That arrangement continues until the president sends another notice to declare that he is once again able to do the job.
The amendment also contains a provision that has never been invoked to cover the eventuality of a president unable to do the job but also unable to notify Congress. If the vice president and a majority of Cabinet members determine that the president is unable carry out his duties, then the vice president becomes acting president.
In addition, the rules and bylaws of the two major political parties govern what would happen if a presidential candidate resigns or dies before the general election, according to John Fortier, an expert on presidential succession at the Bipartisan Policy Center. The national committee of the affected party would choose a new candidate.
If the candidate dies after the election but before the Electoral College meets in mid-December, the national party would again choose a replacement and the electors would vote for that person, Fortier said.
If the death or resignation happens after Congress counts the electoral votes but before Inauguration Day, the 20th Amendment provides that the vice president becomes president.
If both the president and vice president become incapacitated, the Constitution doesn't directly provide the answer. But a federal law, the Presidential Succession Act, fills in that blank.
First in line to serve as acting president is the House speaker, now Nancy Pelosi, followed by the president pro tem of the Senate, who is now Charles Grassley of Iowa, followed by members of the Cabinet.
But Pelosi or Grassley might be reluctant to accept the role, because they would be required to resign from Congress in order to take the position and could serve only until Congress finally decided who the president will be.
Brian Kalt, a professor at Michigan State University College of Law, said the lack of constitutional clarity could lead to problems.
"The speaker could claim that she's in charge under statute but that could be legally challenged, potentially by the secretary of state, who is the next executive branch official in the line of succession," Kalt said.
"There is no precedent in American history for both the president and vice president becoming unable to carry out their duties," added Kalt, author of "Unable: The Law, Politics, and Limits of Section 4 of the Twenty-Fifth Amendment."
"We don't have a process; we don't have standards," he said. "You could have (Secretary Mike) Pompeo coming in and saying, 'Well, I'm in charge.'"
But Fortier doubts it would come to that, saying, "If they both become incapacitated, then we'd follow the Succession Act."