President Barack Obama governed not to lose. It's a virtue, to supporters: Obama's job was proving an African-American could lead the United States (and as such, the planet). How to prove what shouldn't need proving? By being judicious, avoiding large mistakes; by compensating with an occasional gamble (bin Laden); and of course, by avoiding even the perception of favoritism to Americans of color. Mission accomplished (too well, some say).
In choosing President Obama's worst mistake, then, we'd give varied answers: the bailouts, Fast and Furious, the health care site debacle. With inevitable exceptions, however, Obama was masterful at tone-setting, dignified to a fault (while inevitably compared to Lincoln, in presidential style he may be closer to George Washington).
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Effectively, Obama ruled so the next president of color might be (simply) "president." Even in 2008, there were grumbles: a black man takes center seat only with an elder white male as backup/spotter. To avoid doubts over precedence (after Dick Cheney), V.P. Biden had to be typically ceremonial. To his credit, he kept place.
Still, with continued racism among existential threats, Biden's 2020 candidacy is problematic. Unfair or illogical as it may be, his nomination could make Obama seem more, not less, like the beneficiary of a cynical bargain. Obviously, Biden can't be historic in the same way, but a Biden presidency puts Obama in his shadow, at least temporarily. The prospect is exacerbated if Biden is a notably great president, or a bad president or, even, a mediocre president during especially crucial times.
Many resist such esoteric analysis: we (should) choose a leader based on substance, on fitness, not from supposed hypersensitivity over past associations. Nevertheless, the Democratic race, so far, has been characterized by notable hostility to Joe Biden -- arguably, more than expected -- with offense taken to his word and deed.
In this light, even with his bold-print resume, name recognition and confidence to spare, Biden can't be president -- not because he lacks ideas -- not because "handsy" -- from circumstance. The chip on his shoulder, lately, might suggest he knows (his "Wise Mind" knows). Like the gaffe-prone John McCain and Mitt Romney, Joe Biden runs against Barack Obama, if only in the American imagination.