Following his stumbling performance at the first debate against former president Donald Trump, President Joe Biden has sought to reassure the Democratic Party about his electability, as well as his age and capacity to govern.
Yet public calls from some Democratic officials and donors to replace him on the ballot have raised questions of what procedural options there are for doing so.
One of these is the , which governs presidential succession.
Although it deals with a critical issue, the 25th amendment dates only to 1967.
Before then, the only relevant statute was Article II of the Constitution, stating that the vice president would assume presidential duties in the event of the president's death, resignation or inability to discharge his duties.
But this article had major limitations: It said nothing about replacing the vice president, nor did it specify how a president's inability to govern would be determined.
The lack of clarity has caused problems throughout U.S. history — James Garfield and Woodrow Wilson both spent portions of their presidencies largely incapacitated, with no mandate for the vice president to assume office while the president still lived.
After President John F. Kennedy's assassination in 1963, there was a greater urgency to clarify the rules.
Proposed in 1965 by Senator Birch Bayh and ratified two years later, the 25th Amendment contains four sections.
Section 1 reiterates Article II but goes further to clarify that in the event of a presidential vacancy, the vice president becomes the new president and fully succeeds to the office rather than only assuming its duties as acting president. This had previously not been specified and initially caused confusion when John Tyler succeeded William Henry Harrison.
Section 2 filled a critical gap by laying out a procedure for installing a new vice president: nomination by the president and confirmation by both houses of Congress.
This section's importance was proven shortly afterward in 1973, when Vice President Spiro Agnew resigned over corruption charges. Left without a vice president and facing his own investigation over the Watergate scandal, President Richard Nixon would have been succeeded by House Speaker Carl Albert as per the Presidential Succession Act of 1947.
Instead, Nixon invoked the 25th Amendment for the first time to nominate Gerald Ford as vice president. Less than a year later, Ford became president upon Nixon's resignation.
Section 3 allowed a president to declare an inability to govern by formally notifying congressional leaders, thereby temporarily transferring his powers and duties, and allowing the vice president to become acting president.
A president may also reverse this action by notifying Congress of a fitness to resume office.
Previously, a similar arrangement had been informally adopted between President Dwight D. Eisenhower and Vice President Richard Nixon, with Nixon filling in multiple times when Eisenhower was hospitalized.
Since then, the section has been invoked by Ronald Reagan, George W. Bush and Joe Biden when undergoing surgeries.
Section 4 is the most complex part of the amendment, addressing who else besides the president may declare the president unable to perform official duties.
The text grants this power to the vice president, along with a majority of presidential Cabinet members, though it also allows Congress to designate other officers instead.
Congress also has the final say if the president disputes the declaration and maintains a fitness for office, requiring a two-thirds vote to determine that the president is truly incapable of governing.
This section is similar to the impeachment process in allowing a sitting president to be relieved of power by other government officials.
But unlike impeachment, Section 4 only allows a president to be suspended rather than removed from office, with the vice president becoming acting president until the next election.
And while impeachment applies to wrongful conduct or abuse of presidential powers, the 25th Amendment concerns the ability to exercise these powers.
The scope of what constitutes inability, however, is intentionally left open. While the most obvious interpretation is a health emergency, constitutional experts have raised other possibilities such as kidnapping, temporary impairment or extreme incompetence.
In fact, there are few formal limits on what can be considered presidential inability as long as the vice president, a majority of Cabinet members and a supermajority of Congress can agree on it.