Should something happen to the president, there is a process in place to replace him or her and it's called the "presidential line of succession." The "line" is there to insure the continuity of government, meaning someone will be in charge to direct the necessary functions of the federal government.
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How did this come about?
The process of replacing a president falls under something called the Presidential Succession Act of 1792. The act allowed that if the president and the vice president were unable to serve (were killed, for instance), the Senate president pro tempore (the person who presides over the Senate when the vice president is not there) would be sworn in as president. Should something happen to the president pro tempore, the person next in line would be the speaker of he House.
In 1886, the succession order was changed, switching the place of the president pro tempore with that of the speaker of the House. Members of the president's cabinet where then added to the line of succession.
The order was changed once again in 1947. It was decided then that the cabinet members would be in line to assume the presidency by order of the date their offices were established. That made the Secretary of State the first cabinet official on the list.
When did they start choosing a specific person to be the designated survivor?
It was during the Cold War in the late 1950s that specific cabinet members were named as the designated survivor for particular events.
As a side note, after the 9/11 attacks in 2001, congressional members devised their own version of designated survivor. Should all the senators and representatives be killed, the designated survivors from the House and the Senate would become Speaker of the House and President Pro Tempore of the Senate, respectively. They would then be third and fourth in the line of succession to the presidency, after the vice president.
The choice of the person who would replace the vice president if something should happen to him is addressed in the 25th Amendment. The president would nominate a person to be vice president, and that person would take the office if he or she got a majority vote of both houses of Congress.
So who/what is the designated survivor?
The designated survivor is a person chosen to remain physically away from a large gathering that includes the president, vice president and other high-ranking members of the federal government – think State of the Union Speech – in case of an attack on that gathering.
The person chosen as designated survivor – generally a cabinet member – must be in the line of presidential succession as laid out by the Presidential Succession Act and the following acts that amended the original.
The person must also be able, constitutionally, to assume leadership of the country. That means the person must be a naturally-born U.S. citizen, be at least 35 years or older, and have at least 14 years of residency in the United States.
Who could be designated survivors today?
Here’s the current list, in order of designated survivors:
1. Vice President - Joe Biden (D)
2. Speaker of the House of Representatives - Paul Ryan (R)
3. President pro tempore of the Senate - Orrin Hatch (R)
4. Secretary of State - John Kerry (D)
5. Secretary of the Treasury - Jack Lew (D)
6. Secretary of Defense - Ash Carter (D)
7. Attorney General - Loretta Lynch (D)
8. Secretary of the Interior - Sally Jewell (D)
9. Secretary of Agriculture - Tom Vilsack (D)
10. Secretary of Commerce - Penny Pritzker (D)
11. Secretary of Labor - Thomas Perez (D)
12. Secretary of Health and Human Services - Sylvia Mathews Burwell (D)
13. Secretary of Housing and Urban Development - Julián Castro (D)
14. Secretary of Transportation - Anthony Foxx (D)
15. Secretary of Energy - Ernest Moniz (D)
16. Secretary of Education - John King (D)
17. Secretary of Veterans Affairs - Bob McDonald (R)
18. Secretary of Homeland Security - Jeh Johnson (D)
Note: Secretary of the Interior Sally Jewel is not a naturally-born U.S. citizen, therefore she could not become president.
Who chooses the designated survivor?
Generally, the choice is made by the president’s chief of staff.