Joe Biden knew that history was stacked against him, acknowledging candidly Wednesday that the “window” for him to seek the Democratic nomination “has closed.”

After his son died of cancer, people close to the vice president said all summer that the biggest hold up in his thinking was the question of whether he and his family had the emotional wherewithal to run a national campaign.

Flanked by President Obama in the Rose Garden, Biden said his family decided they had that energy. In deciding not to run, then, he bowed to reality that followed a sober-minded assessment of the facts on the ground in the early states, the hurdles of building an operation and, significantly, the history of vice presidents seeking a promotion to the top job.

“Unfortunately, I believe we’re out of time, the time necessary to mount a winning campaign for the nomination,” the vice president said Wednesday.

Biden ran unsuccessfully for president in 1988 and 2008. He seriously considered running in 1984 and 2004 but didn’t go through with it – at the last minute, in both cases.

Biden is a keen student of history. The day before he revealed his decision, he spent an hour reminiscing with former Vice President Walter Mondale about the men who have held the job. Slouched on a couch at George Washington University, Biden cited historian Robert Caro to recall how frustrated Lyndon Johnson was with  John F. Kennedy’s vice president and then, when he succeeded Kennedy, how he marginalized his own vice president, Hubert H. Humphrey.

As his months-long Hamlet act wore on, Biden made clear that he was thinking about his legacy. Undoubtedly, if he had run and lost, it would have been tarnished. It would have been hurt even more if, in losing, he bloodied the eventual Democratic nominee enough to cost her, or him, the presidency.

Only four vice presidents have ever been elected directly to the presidency: John Adams, Thomas Jefferson, Martin Van Buren and George H.W. Bush.

Another cautionary tale from Biden’s own lifetime also clouded his hopes. When he was nine-years-old, in 1952, Harry Truman’s vice president tried to run in the race to succeed him and failed disastrously. Major factors in Alben Barkley’s failed run for the Democratic nomination were that he waited too long to get in, that he was perceived as too much a part of Washington and that he was widely seen as too old.

Biden had all three of the same problems.

Each of them served 36 years in Congress when tapped by a younger presidential candidate to be his running mate, largely because of experience and relationships on the Hill.

Barkley was 74 when he ran for president; Biden is 72.

One of the biggest potential problems for Clinton is that voters are fatigued after watching her for 25 years. But Biden has been on the national stage for 43 years. He was elected to the Senate in 1972, the year that Richard Nixon won his second term.

Back in 1952, Tennessee Democratic Sen. Estes Kefauver, a 40-something whippersnapper, had made a name for himself leading televised hearings into organized crime. But he rubbed a lot of party leaders the wrong way, and they wanted to unite behind an alternative after he won the New Hampshire primary and became the front-runner.

Barkley thought he could be a white knight, swooping in to save the day, but the establishment never coalesced around him. He was from Kentucky, but he could not even get support from the South. Richard Russell, the Georgian who now has a Senate office building named after him, won the Florida primary. Minnesota Sen. Hubert Humphrey, pushing for civil rights, was strong in the North but despised in Dixie. Averell Harriman, who had been an ambassador and Commerce secretary, siphoned off more establishment support and won the West Virginia primary.

It was a time when party bosses still largely controlled state delegations and national conventions really mattered, but as the sitting vice president Barkley could still not win a single state’s primary. Democrats eventually turned to Illinois Gov. Adlai Stevenson, himself the grandson of Grover Cleveland’s vice president.

Biden was in a somewhat similar boat. As he called around to old colleagues, who have been friends for decades, it became clear that the only senators whose endorsements he could count on were from his home state of Delaware.