If President Donald Trump or Vice President Mike Pence are somehow forced out of office, the next man up in the presidential line of succession will be House Speaker Paul Ryan.
In July 1947, President Harry S. Truman signed the Presidential Succession Act. The act came about after Truman, who had succeeded President Franklin D. Roosevelt after his death in April 1945 and would go on to win his own term in 1948, suggested that an elected officials be put ahead of cabinet members.
It was decided to place the Speaker of the House and the President pro tempore of the Senate after the Vice President. Both are elected positions.
The President pro tempore is the second-most senior position in the Senate. He or she is appointed by the Vice President, who serves as the President of the Senate and can cast the tie-breaking vote. The President pro tempore is typically the longest-serving Senator of the majority party. Therefore, Senator Orrin Hatch of Utah, who has been in the Senate since 1977, currently holds the position.
After the Vice President, House Speaker and President Pro Tempore of the Senate, the line of succession is made up of the un-elected members of the cabinet. They are placed in the order the office was created. So, Secretary of State Rex Tillerson is fourth on the list, while Acting Secretary of Homeland Security Elaine Duke is 17th.
The rules of the Constitution on who is eligible to serve as President are still in place. You still have to be at least 35 years old and a natural-born citizen of the U.S. This means Transportation Secretary Elaine Chao, the wife of Senate Majority Leader Mitch McConnell, is ineligible as she was born in Taiwan.
The Constitution originally didn't specify if a presidential successor became president or just Acting President. When Vice President John Tyler succeeded President William Henry Harrison in 1841, he insisted that he was not an Acting President, but the president with full powers. It wasn't until the 25th Amendment was adopted in 1967 that this was set in stone. As Section 1 reads, "In case of the removal of the President from office or of his death or resignation, the Vice President shall become President."
However, in the history of the U.S., the only President who left office without being assassinated was President Richard M. Nixon, who resigned in 1974.
The 25th Amendment also gives Congress the power to fill a Vice Presidential vacancy in the second section. Section three allows a president to write to the House Speaker and the president pro tempore of the Senate to delegate presidential duties to the Vice President. Section four allows the Vice President and a majority of cabinet members or Congress to declare the president "unable to discharge the powers and duties of his office."