In 1947, the new Republican controlled 80th Congress, the first Congress to have both houses being Republican controlled since 1928, acted in revenge against the memory of Democratic President Franklin D. Roosevelt by changing the Presidential Succession Act of 1886.
That law in 1886 made the succession to the Presidency to be the cabinet officers after the Vice President, including in order, the Secretary of State, Secretary of the Treasury, Secretary of War, Attorney General, Postmaster General, Secretary of the Navy, and the Secretary of the Interior. The original law in 1792 made the President Pro Tempore of the Senate and the Speaker of the House next in line before the cabinet members, and after the Vice President.
This was changed to the present situation in 1947, that the Speaker of the House of Representatives, and then the President Pro Tempore of the Senate would succeed after the Vice President and before the cabinet officers.
This has led to people in the line of succession who, much of the time, have been the opposition party to the President, as in 1947-1949 under President Harry Truman; 1955-1961 under President Dwight D. Eisenhower; 1969-1974 under President Richard Nixon; 1974-1977 under President Gerald Ford; 1981-1987 in the House under President Ronald Reagan; 1987-1989 under President Ronald Reagan; 1989-1993 under President George H. W. Bush; 1995-2001 under President Bill Clinton; 2001-2003 in the Senate under President George W. Bush; 2007-2009 under President George W. Bush; and 2011-2017 under President Barack Obama.
This is not proper, to have the opposition party have the potential to take over the Presidency through having a Speaker of the House and/or a President Pro Tempore of the Senate of their party, rather than having the continuity of the administration though the cabinet members chosen by the President.
So 44 years between 1947 and 2017, out of a total number of 70 years, or just about two thirds of the time, the opposition party has been two heartbeats away from the Presidency, undermining continuity of government.
Also, just because someone is Speaker of the House (elected by one Congressional district) or President Pro Tempore of the Senate ( an often very old person in that position, elected from one state, who has longevity of service) does not make such a person qualified to be President, as much as a Secretary of State, Treasury, etc does!
So while it is unlikely to happen anytime soon, there really is a need to change the Presidential Succession Act back to the one passed and in effect from 1886-1947!