And it’s spelled out here:
All of the Democratic Vice-Presidents that Biden grew up watching eventually became Democratic nominees for President: Lyndon Johnson, Hubert Humphrey, Walter Mondale, and Al Gore. If Biden decides to forego a run for the Oval Office, he will be the first Democratic Vice-President unable to secure his party’s nomination in sixty-three years. (In 1952, the seventy-four-year-old Alben Barkley, Harry Truman’s veep, was pushed out of contention by party leaders who considered him too old. Biden is seventy-two.)
Of that list, only Lyndon Johnson became President, and LBJ was elected frankly because in 1964 the reflected glory of the martyred JFK still shone upon him. If Kennedy had lived, Johnson might have run in 1968… and he would have lost. Everybody else on that list lost. Even Al Gore barely squeaked out a popular vote win (while losing the Electoral Vote – and his own state, which is impressive, in its way).
Mind you, the author of that piece is barely concealing his panic that Hillary Clinton is on a nice, steady glidepath to a crash-landing right on top of the Marianas Trench. But that’s just life in the Imperial Capital. We got lots of courtiers here who don’t like surprises…