What’s driving all three men — who’d all otherwise be well into their retirements — to stay engaged? Experts cite ego as a major factor for both Biden and Trump.

“The fact they both made it to the White House makes them think they both know how to get to the White House,” said Brinkley. “Once you have power, it’s very hard to let it go.”

Biden’s insistence on staying in office also likely stems from his own awareness of history. In particular, he has a longstanding fear that Democrats nominating a more liberal candidate such as Sanders would mean giving Republicans a better shot at winning the White House.

“The problem with the Democratic party is George McGovern syndrome,” said Brinkley, referencing the anti-war nominee Democrats ran in 1972 against the incumbent GOP President Richard Nixon. McGovern won a single state — Massachusetts — plus the District of Columbia. Incidentally, 1972 is the same election cycle when Biden, at age 29, was first elected to the US Senate.