The 25th amendment and the 4 key sections

section 2

if the Vice Presidency becomes vacant, the President may choose a new Vice President, who must be voted and approved by Congress before the candidate can have the position

25th amendment

presidential succession in case something happens to the president

section 3

the president may temporarily make the Vice-President the Acting President with a written declaration. This lasts until a second declaration is made that ends this condition. This has been done a few times, most recently when President Bush underwent two medical procedures in 2002 and 2007 and passed power to Vice President Cheney.

section 1

the process where the Vice President becomes President if the current President dies, resigns or is removed from office becomes official.

section 4

this is an emergency provision that allows the Vice President and members of the Cabinet to declare the President unfit to carry out the duties of the Presidency. The President may assert his competency (ability to serve) by sending a declaration to Congress. The Vice President and Cabinet can submit another declaration against the President, which would force Congress to reach a 2/3 majority that the President is unfit for office. Section 4 has never been used, but Sections 1 - 3 have all been used at one time or another.