Mr. Buttigieg and Ms. Klobuchar, for instance, have virtually no support among black voters in national polls. Mr. Sanders has substantially more support among black voters and could easily claim the lead among them in the next round of national polling. His vast financial resources and status as a well-known returning runner-up give him some of the advantages that establishment-backed candidates have usually relied on to outlast activist-backed candidates.

The apparent decline in Elizabeth Warren’s standing has also allowed Mr. Sanders to consolidate the party’s progressive left. The calendar offers few opportunities for her to regain her footing; the same is true for the other candidates with predominantly white, well-educated support. She will have to create her own magic, probably on the debate stage, the way Newt Gingrich did ahead of the South Carolina primary in 2012 or the way Ms. Klobuchar did last week.

But despite the advantage of a potentially unified left, Mr. Sanders does not seem like a juggernaut poised to roll to the nomination, at least not yet. He has somewhat underperformed his final poll numbers in both Iowa and New Hampshire. It would be wrong to assume that all of the moderate voters would coalesce behind a single moderate candidate, but there’s no doubt that the more moderate candidates, combined, have fared better than the sum of Mr. Sanders and Ms. Warren.

Party math and the magic 15

Most important, the Democratic nomination rules, which award delegates fairly proportionally among candidates who exceed 15 percent of the vote in a state or district, make it hard for him to win a majority of delegates on Super Tuesday with a plurality of the vote.

Take Donald J. Trump in 2016 as an example. At this point in the race, he had a wider lead in national polls than Mr. Sanders does, won New Hampshire by a wider margin and would soon win Nevada and South Carolina by wider margins than seem likely for Mr. Sanders at this point. He then won a decisive victory on Super Tuesday. Yet under the Democratic delegate rules, he would have been left well short of a majority of delegates, potentially setting the stage for a contested convention.

There are some situations where Mr. Sanders might nonetheless rack up a big delegate majority: if he is the only candidate who breaches 15 percent of the vote in a state, or if only he and one other person do so. This is possible; the non-Sanders candidates who are over 15 percent are generally in decline, while some of those on the rise are well beneath 15 percent.

Even if three candidates get over 15 percent nationwide, the real key is whether three candidates will be over 15 percent in every state, as at least three candidates were in every state in the Republican contest on Super Tuesday in 2016. The difference between whether one or five candidates breach viability in a Sanders-friendly state like California might wind up being pretty narrow, and the whole nomination could turn on it.