What if Trump Can’t Run? Many Steps Are Clear, but Some Are Not

“Continuity of government is always in place,” she said on MSNBC. “I always say it’s a relic of the past, but nonetheless, they say we have our job we have to do, and this is what we’ll do.”

Senator Charles E. Grassley, Republican of Iowa, is the current president pro tempore in the Senate. He is 87.

Under the 25th Amendment, ratified in the 1960s to clarify presidential disability and succession planning, presidents can voluntarily designate powers to their vice presidents if they become seriously ill or are unable to perform their duties.

If Mr. Trump became gravely ill, he could provide letters to the speaker of the House and the president pro tempore of the Senate saying he was “unable to discharge the powers and duties of his office” to transfer his powers to Mr. Pence, who would, in effect, become acting president. Mr. Trump could reclaim his full authorities when he recovered.

On Friday afternoon, as Mr. Trump was expected to depart for Walter Reed National Military Medical Center, Judd Deere, a White House spokesman, said the president would remain fully in power.

“No transfer,” he said. “The president is in charge.”

Since the amendment was ratified in 1967, the vice president has taken power in only three instances, each of them exceedingly brief. In 1985, when President Ronald Reagan was put under anesthesia for a colon procedure, he granted his powers to Vice President George Bush for about eight hours, though he avoided formally invoking the amendment. And in 2002 and 2007, President George W. Bush temporarily transferred his authorities to Vice President Dick Cheney during colonoscopies.

The 25th Amendment also allows for the forcible removal of a president, including if he is too ill to designate his authorities or simply refuses to do so.