According to the RealClearPolitics average of the national polls, President Biden’s approval rating currently stands at 55%, a fine position overall but not a great place to start for a new president. In recent history, only Donald Trump had weaker numbers upon ascension to the White House. Yet Biden’s political prospects over the next months and years are far from certain. A lot will depend, of course, on how well the economy recovers from the coronavirus pandemic and, relatedly, how much longer the pandemic continues. But much will also hinge on how well he can manage the great diversity of interests within the Democratic Party. The common way to conceptualize the Democratic Party is along an ideological spectrum, from centrists such as Janet Yellen and Joe Manchin in the middle to progressives such as Alexandria Ocasio-Cortez and Bernie Sanders on the Left. While there is a lot of merit to this framework, it is also useful to think of the party as a collection of factions, each of which has its own interests to which party leaders must be responsive. Parties are, by their nature, factional, in the sense that for a country as big and diverse as ours, but with only two parties, there are by necessity partisan alliances among different subgroups. But the Democratic Party has long been particularly noteworthy for its hodgepodge quality. After the Civil War, for instance, the party was...