The Presidential Succession Act was changed in 1947 from what it had been in the earlier law of 1886.
Instead of the cabinet officers being next in line after the Vice President, the new law, in effect now for 68 years, has the Speaker of the House of Representatives, a Congressman elected by one Congressional district, as next in line.
So therefore, three Speakers of the House have been “a heartbeat away” from the Presidency, in mid 1947-1948, November 1963 to January 1965, and October to December 1973 and August to December 1974.
Joseph W. Martin Jr. was the first Republican Speaker in 16 years, when the law changed, and when threats against Harry Truman by the Zionist Stern Gang in 1947, as reported by Margaret Truman, occurred, and Martin was a heartbeat away.
When President John F. Kennedy was assassinated on November 22, 1963, and Lyndon B. Johnson, who had suffered a heart attack in 1955 became President, 73 year John W. McCormack was next in line for 14 months, and the recognition of this fact and his advanced age, led to the passage and ratification of the 25th Amendment in 1967, providing for an appointed Vice President to fill a vacancy after hearings by the House of Representatives and Senate.
Carl Albert was the third Speaker to be next in line when Vice President Spiro Agnew resigned in October 1973, and Albert remained so for two months until Gerald Ford was selected and confirmed as the the first Vice President under the 25th Amendment.
Again, Albert was first in line from August 1974, when Richard Nixon resigned and Gerald Ford became President, until December 1974 when Nelson Rockefeller was selected and confirmed as the second Vice President under the 25th Amendment.
So for a total of about two years, we have had Speakers of the House, and all three of the opposition party to boot, as “a heartbeat away” from the Presidency.
And although no President or Vice President has left office since 1974, the odds of such an event occurring at some point in the future is mounting, and worrisome, with three out of four years since 1947 having the opposition party in the Speakership as two heartbeats away from the Presidency!