evidence: It is not unprecedented for an incumbent president to abandon his reelection campaign. But it is rare, and no president until President Joe Biden has been pressured out of a reelection campaign out of concerns about his mental fitness. One warning bell for Democrats who assume that a nominee younger than Biden – he has endorsed Vice President Kamala Harris – would do better against former President Donald Trump: Republicans won the two most recent elections in which presidents eligible for reelection bowed out of their campaigns. [...] Biden was never under any real pressure from Democrats to stand down earlier this year, and he faced only token opposition for the Democratic nomination. Nearly all of the delegates to the Democratic National Convention are pledged to him. In other words, Truman and Johnson both avoided a bruising primary by ending their reelection campaigns. Biden had the Democratic nomination sewn up, and he chose to give it up.