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...