I'm sure I'm not the first person to have noticed that in the last century those Vice Presidents who later run for the presidency do not fare especially well. Some fail to get elected, some get elected but cannot get reelected, some have decided not to run for reelection, and some have resigned.
Franklin Roosevelt's last VP Harry Truman served most of FDR's fourth term, and was elected once to the office, but didn't run for reelection though he was eligible to do so. History treats him kindly but he was quite unpopular when he left office.
Eisenhower's VP Richard Nixon ran but was defeated by Kennedy. Kennedy's VP Lyndon Johnson assumed the office when Kennedy was killed, won election to continue in office but, like Truman, decided not to run for reelection. Probably for the same reason, too.
Nixon ran again following Johnson, won, and then won reelection but resigned because of Watergate. Nixon's second VP Gerald Ford assumed office when Nixon resigned but could not get elected when he subsequently ran for the presidency. Carter's VP Walter Mondale ran for president but was defeated.
Reagan's VP George H.W. Bush was elected POTUS but failed to get reelected. Clinton's VP Al Gore ran for president but was defeated. Barack Obama's VP Joe Biden was elected but couldn't get renominated by his party. Trump's first VP Mike Pence could not secure his party's nomination.
Biden's VP Kamala Harris inherited the nomination but was defeated. No former VP has served two full consecutive terms as president in the last century.
The vice presidency appears to be an inherently unlucky office. Yet people continue to volunteer for the job that former VP John Nance Garner confided to fellow Texan LBJ "wasn't worth a bucket of warm spit."
N.B., I'd guess "spit" was something equally biological, but even less attractive, in the original.
