The Twenty-Fifth Amendment

"Section 1. In case of the removal of the President  from office or of his death or resignation, the Vice President shall become President.

Section 2. Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress.

Section 3. Whenever the President transmits to the President pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that he is unable to discharge the powers and duties of his office, and until he transmits to them a written declaration to the contrary, such powers and duties shall be discharged by the Vice President as Acting President.

Section 4. Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President.

Thereafter, when the President transmits to the President pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that no inability exists, he shall resume the powers and duties of his office unless the Vice President and a majority of either the principal officers of the executive department or of such other body as Congress may by law provide, transmit within four days to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office. Thereupon Congress shall decide the issue, assembling within forty-eight hours for that purpose if not in session. If the Congress, within twenty-one days after receipt of the latter written declaration, or, if Congress is not in session, within twenty-one days after Congress is required to assemble, determines by two-thirds vote of both Houses that the President is unable to discharge the powers and duties of his office, the Vice President shall continue to discharge the same as Acting President; otherwise, the President shall resume the powers and duties of his office."

Something is happening in this picture that no one will ever see, that is, unless you are looking for it.  It is the Twenty-Fifth Amendment in full motion.  The Twenty-fifth Amendment was ratified on February 23rd, 1967.  During the State of the Union Address, which happens once every year, one member of the Presidential Cabinet is always selected to stay back at the White House.  Here, said person is under heavy protection under the Secret Service.  God forbid something should ever happen during the State of the Union, but if all the members of the line of succession to the President were to die including the President himself, the person left at the White House would immediately be sworn into office as the President of the United States.  This line of succession is fully established within the Twenty-Fifth Amendment. 

As of 2008, this was the Presidential line of succession.  Should the President die while in office, the Vice President would take over his office.  Should the President die in office while there is no current Vice President, the Speaker of the House would take over.  It is important to note that members of the line of succession do not simply move up when a member ahead of them dies.  For instance, the Secretary of Education does not become the Secretary of Energy simply because the Secretary of Energy dies.  A new replacement for vacant position is immediately filled.