Fourth, the white college graduates — gentry liberals — who are, for the first time in history, one of the Democratic Party’s largest constituencies, are a fickle bunch.

Black and elderly Democrats have consistently given former Vice President Joe Biden large pluralities, and Hispanic and low-income non-college Democrats have shown some affinity for Sanders. That largely accounts for the buoyancy of support for these 77- and 78-year-old candidates.

But gentry liberals have been bouncing around. They were briefly smitten with Harris after she bopped Biden on school busing. They swooned longer for Warren when she kept repeating, “I have a plan for that,” and then they were charmed by Mayor Pete Buttigieg’s crisp and self-assured articulateness.

The gentry liberals’ fling with Harris didn’t last long, and current polling suggests their crushes on Warren and Buttigieg are over. But there’s still plenty of room for these voters to swing decisively in February’s first two contests, for they are numerous among those who bother to attend the Iowa caucuses and demographically a large share of the population of New Hampshire.