Barack Obama thinks about his brand and his legacy a lot, and he dreads what Joe Biden might do to it. This month alone, Biden mistook New Hampshire for Vermont even though the sole reason he was in that part of the country was the first-in-the-nation primary, which is famously not held in Vermont. He said RFK was assassinated in the 1970s. He said he couldn’t remember the name of the British prime minister in the midst of bragging about his foreign-policy chops. And he speculated about what it might have been like if Obama had been assassinated.This last, ghastly gaffe might be too much for Obama to take. Obama believes his brand is cool, technocratic, tightly controlled, a bit wonkish and professorial, inspirational, buttoned-down, Ivy League, definitely forward-looking. Biden is warm, free-wheeling, intellectually ungifted, frank and unguarded to the point of outright sloppiness, so old he served in the Senate alongside segregationist Democrats, and so out of touch he actually touts...