Both candidates for U.S. president have now picked their running mates. I won’t comment on the veep nominees except to say Barack Obama and John McCain clearly made their choices only to exploit Joe Biden’s and Sarah Palin’s popularity in their respective home states to rake in the electoral mother lode that Delaware and Alaska represent (both states bring in a whopping three electoral votes).

The Palin and Biden selections, however, should make us think about the vice presidency for a bit. Remember a year ago, as the possible candidates for president started lining up, many people talked about how this was going to be an especially unique election because it was the first time since 1968 that neither an incumbent president nor an incumbent vice president would be one of the major parties’ nominees? The field was wide open in ways we haven’t seen in decades, the pundits said. Some questioned George W. Bush in 2004 for sticking with Dick Cheney—who has always said he had no intention of running for president—and not grooming a successor. It is an interesting point since lots of vice presidents have succeeded their bosses, but when we look at recent history, it is actually rare that a sitting vice president takes over the executive branch via the electoral process. In fact, it happened only once since 1900.

Theodore Roosevelt, Calvin Coolidge, Harry Truman, Richard Nixon, Lyndon Johnson, Gerald Ford, and George H. W. Bush were all vice presidents who later became president. Roosevelt, Coolidge, Truman, and Johnson all started their first terms as president when their predecessors died while in office—oddly enough, they also all won reelection after finishing out the previous term. Roosevelt took over William McKinley’s term after McKinley succumbed to complications from the gunshot he received in an assassination attempt. Coolidge became president when Warren G. Harding died from either a heart attack or a stroke. Harry Truman succeeded Franklin Delano Roosevelt when Roosevelt passed away due to a cerebral hemorrhage. Lyndon Johnson became president after John F. Kennedy’s assassination in Dallas.

Nixon was Dwight D. Eisenhower’s vice president and later won the election to become president, but it was not as Eisenhower’s successor. He actually failed in his attempt to follow Eisenhower by losing to Kennedy in 1960 and did not win a presidential election until 1968. Since 1900, vice presidents who actually ran after their tenure as second in command—the number is actually pretty low—it was more often the case that they lost.

Ford succeeded Nixon after Nixon resigned, but unlike the presidents who began their first term by inheriting the office after the predecessor’s death, Ford did not win reelection.

That means since 1900 only one vice president has actually succeeded their outgoing boss by election. In 1988 George H. W. Bush won the election for president as Ronald Reagan’s sitting vice president.

How much weight then, do presidential nominees and their parties give to trying to create a line of succession when they choose the running mates? It seems that hoping to create legacy via lineage would not play as much of a factor as the other factors do—shoring up weaknesses, trying to gain an electoral advantage in a state or region, etc.