As should be pretty intuitive, larger fields are correlated with more prolonged nomination processes in which both voters and party elites have a harder time reaching consensus. Below is a table comparing the number of candidates in each past cycle against the share of the overall popular vote the nominee eventually received. I’ve also included a more subjective measure of whether party elites were able to get their way. I consider the party to have decided — that is, for party elites to have gotten their preferred choice — if there was a clear front-runner in endorsements in advance of the Iowa caucuses and that candidate won the nomination. And I consider the elites to have failed if a factional candidate who lacked broad support from the party establishment won. Then there are in-between cases such as the 2008 Democratic primary, in which party elites didn’t necessarily get their first choice (or there wasn’t a clear first choice), but the candidate who emerged was broadly acceptable to multiple major factions of the party.
This table ought to worry establishment Democrats. The three past elections when the field was as large as its shaping up to be in 2020 all resulted in party elites failing to get their way. They also resulted in a nominee who failed to get 50 percent of the popular vote in the primaries, which could yield a contested convention since Democratic delegate allocation rules are highly proportional to the popular vote. In a field of 20 candidates, for instance, you’d project — extrapolating from the data above — that the eventual nominee would have either 32 percent or 40 percent of the popular vote, depending on whether you use a linear or logarithmic trendline. That could mean that the race is decided at the convention.