FILE - President Richard Nixon, left, and Vice President Gerald Ford chat in the Oval Office as they go over papers, Dec. 1973. Of the 49 vice presidents in U.S. history, nine of them — or nearly 1 in 5 — have risen to the presidency due to death or resignation. (AP Photo, File)
FILE - This photo taken April 10, 2014, shows a 1841 oil portrait of President William Henry Harrison. Of the 49 vice presidents in U.S. history, nine of them, or nearly 1 in 5, have risen to the presidency due to death or resignation. (Jenny Peter/The Sun Commercial via AP, File)
FILE - In this 1904 file photo, Theodore Roosevelt campaigns for the presidency in 1904. Roosevelt became the youngest person to serve as president at 42 years old when William McKinley was shot and killed in 1901.(AP Photo, File)