By contrast, the Democratic Party’s egalitarian-minded rules allocate all pledged delegates to its convention on a proportional basis: A presidential candidate who receives at least 15 percent of the vote in any state or congressional district receives a corresponding share of delegates, making it difficult for a leading candidate to become a runaway train. In fact, had the 2016 Republican primary played out under Democrats’ rules, it would have almost assuredly resulted in an ugly, contested convention.

A brokered convention wasn’t a grave concern for Democrats in 2008 or 2016, because those races distilled to two main candidates quickly, virtually guaranteeing one would win a delegate majority. But for 2020, Democrats' jam-packed field is already on track to surpass the Republican 17-way rumble of 2016 and lacks an obvious front-runner. At the dizzying pace small- and large-dollar donors are bankrolling their favorite hopefuls, many Democrats could have the financial wherewithal — and even pressure from their backers — to campaign deep into the primary calendar, dramatically increasing the odds no candidate will capture a majority by the convention. Translation: Democrats could still be fighting among themselves little more than three months before the general election.

Democrats’ increasingly front-loaded primary calendar only adds to the chaos. California and Texas — the two largest states in the country — have moved up their primaries to Super Tuesday, on March 3. This means 36 percent of Democrats’ 3,768 pledged delegates will be allocated in early March, before the herd has truly been culled, making it even harder for one candidate to build a delegate majority. And if Colorado, Georgia and New York decide to join the Super Tuesday stampede, that share could rise to a whopping 46 percent.

Finally, consider the third rail of Democratic primary politics: superdelegates, the unpledged party leaders and elected officials who have automatically been seated at the convention in the past . In 2016, Mrs. Clinton won a clear majority of pledged delegates in the primaries and didn’t need these party elders’ help to prevail. But a common misconception among supporters of Bernie Sanders that this collective of “insiders” robbed him of the nomination played right into Mr. Trump’s assertion that Democrats had “rigged” the primaries for Mrs. Clinton.

In 2020, Democrats have sought to tamp down the superdelegate hysteria by barring these leaders and officials — currently 765 of them — from casting votes on the initial ballot at the convention. But here’s the ultimate irony: They can still cast votes on successive ballots, so they could be more influential than ever if the Democratic primary devolves into a floor fight. And the potential for back-room deal-making or heavy-handed Democratic National Committee refereeing could only further fuel grass-roots suspicion that the party’s elites are running the show, setting ablaze the prospect of party unity.