First, the results today will tell us what the two main coalitions really look like. The first four small states offered clues, but the total number of voters in the 14 Super Tuesday states is exponentially larger. Sanders seems to have failed to grow his support among African Americans much beyond his 2016 percentages, but he has made inroads among Latino voters. He remains dominant among young voters and does better on the coasts and in the northeast. Biden is the candidate of African Americans, suburban moderates, the South, and older voters.

Many of the differences between these two groups are cultural, not ideological. But inside the Democratic Party there is a debate not unlike the one that divides the two main parties about the breadth of change that Washington should pursue. The Democrats’ moderate wing, which is now anchored by older black voters in the south, remains deeply skeptical of Sanders-style socialism, while the New New left, powered by young radicals in big cities, is repelled by the incrementalism of Biden.

This divide between Sanders’s and Biden’s bases might not be easily bridgeable, and if a clear delegate winner fails to emerge, the party’s convention in Milwaukee could be as messy as anything since 1968, when supporters of anti-war candidate Eugene McCarthy took to the streets to protest the establishment-led victory of vice president Hubert Humphrey. How the eventual nominee wins the nod, and how he (or she) handles the inevitable bruised feelings in the other camp, will matter more this year than it has in decades.