National parties are, by their nature, fractious coalitions of competing ideologies and interest groups. Yet even by normal standards, the coalition that makes up the Democratic Party of 2020 is among the least coherent in recent history. If Joe Biden wins in November, a party that represents the will of a stagnated professional middle class while claiming to speak for disparate minority groups and the poor while leaning on the uber-wealthy for funding and messaging could be tasked with creating a governing agenda in 2021. Atop it all will be an anachronistic Washington deal-maker and elder statesman mistrusted by the party’s base who became the nominee in large part because of that very distaste for radicalism. That is not to say Biden himself has a coherent worldview. As party leader, Biden hopes to be all...