Who will make the "final four" of the Democratic field?

The next slot includes a hedge: either Sanders or Warren, but not both. There are enough pure progressives in the party to guarantee one, but not two. While Sanders enjoys about twice the support of Warren (16.7 percent for Sanders in the RealClearPolitics average versus 8.2 percent for Warren), my hunch is that in 2020, wonkiness might edge out anger. Watch closely the primary in New Hampshire, next door to his Vermont and her Massachusetts.

The third slot should go to Harris. Hailing from California, she could win a huge bloc of delegates from her home state. Demographics could help her as well: A quarter of the Democratic primary electorate will be African-American, and women are likely to represent about 60 percent of the Democratic primary vote.

The final, most subjective, slot would be Buttigieg’s. Yes, a 37-year old gay mayor of the 306th largest city in America, seems a stretch. South Bend, Indiana is a bit smaller than Davenport, Iowa, and bit larger than Vista, California.

But the world of American politics has changed.