2. Through the 25th Amendment: A section of this Amendment empowers the President's Cabinet by majority vote to transfer the powers of the President to the Vice President or absent a Vice President, to the person next in line. If the President challenges this decision, it falls to Congress to decide whether to restore the President to power; in the absence of a two-thirds negative vote in both house, the President is back in power. This has never happened.
There were several reasons this Amendment was drafted: first, as a contingency should the President be incapacitated (thus unable to resign); second, if the President were to be captured or kidnapped and unable to act (which could have happened to James Madison when the British captured Washington, DC) or, third, in the Night at Camp David scenario (the name of a best-selling novel) where there is a concern that the President may not be mentally capable of fulfilling his duties. Another provision of the 25th Amendment allows the President to voluntarily step aside (for example, if he or she were to undergo surgery). In the television series West Wing, the President invoked the 25th Amendment when his daughter was kidnaped to insure that someone not emotionally compromised would be making the necessary decisions