In 1947, the Presidential Succession Act was changed from cabinet members to the Speaker of the House of Representatives and the President Pro Tempore of the Senate being right behind the Vice President of the United States.
Since then, as a result of the change of the law, there have been two circumstances involving Presidential assassinations and threats, which provided for the possibility of the Speaker of the House of Representatives becoming President.
The first was Republican Speaker Joseph W. Martin, Jr of Massachusetts in late 1947. The Zionist Stern Gang group, terrorists fighting for the creation of an Israeli nation, had been engaged in violence and assassinations against the British and their control of Palestine.
Margaret Truman, the President’s daughter, claimed her father, President Harry Truman, was threatened with death by letter bombs sent by the Stern Gang to Washington, DC, which were intercepted without harm by the Postal Service, with the assistance of the Secret Service. Had Truman been harmed, however, we would have a switch of parties in the White House, and the second Speaker to become President, with the first being President James K. Polk in the Presidential Election of 1844.
When President John F. Kennedy was assassinated in November 1963, Vice President Lyndon B. Johnson, who had suffered a massive heart attack in 1955, became President, and had he died in office over the next 14 months, Democratic Speaker of the House John W. McCormack of Massachusetts, who was 73 years old, would have become President.
A Secret Service revelation in recent years was that President Johnson was in danger of being shot by an agent overnight after the Kennedy assassination, when he walked out of his home after midnight, leading to an agent, who thought there was an intruder on the property, to withdraw his firearm and aim it until he realized it was the President. The story is that Johnson backed up in shock, but imagine if he had been killed about ten hours after becoming President! And then we would have had the oldest President in American history, Speaker McCormack, an unwilling heir apparent!
These incidents make one wish that the Presidential Succession Act be returned to the 1886 law, making the cabinet members of the President, starting with the Secretary of State, next in line after the Vice President. But that is unlikely to happen, particularly while the Republican Party controls Congress!