Taken together, it is now obvious that a strong opponent to Mr. Sanders has emerged, and it is much harder to see Mr. Sanders claiming an insurmountable delegate lead on Super Tuesday. If so, the big question of one week ago has been answered: The suspense will go on after Super Tuesday.

Of course, broader uncertainty about the state of the race remains. The new question is: Just how far can Mr. Biden surge? The fragmentary and not always high-quality post-South Carolina polling data certainly shows Mr. Biden making big gains, whether nationwide or in Virginia.

But it remains to be seen whether those gains will be enough for him to pull off a symbolically important win outside the several Southern states where he’s favored — most likely in Texas, but perhaps in Maine or, less likely, Minnesota or Massachusetts. To accomplish this, Mr. Biden would probably have to pry away support from Mr. Bloomberg, the one other moderate who remains in the race.

Super Tuesday is the first test of Mr. Bloomberg’s support. State polls suggested he was positioned to reach 15 percent in many Southern contests heading into South Carolina. This would cost Mr. Sanders delegates, and therefore keep him farther from the majority of delegates necessary to win without a contested convention. Yet at the same time, if Mr. Bloomberg’s strength is confined to the South, where Mr. Biden is now poised to do well, Mr. Bloomberg could cost Mr. Biden more delegates than he costs Mr. Sanders.

Recent polls of likely Democratic voters in Super Tuesday states give reason to think Mr. Bloomberg’s voters could break for Mr. Biden. Some of those polls, taken by the survey research company Dynata in collaboration with The Times, and matched to voter file data from L2, suggest that Mr. Bloomberg held nearly three times as much support among voters over age 65, a group skeptical of Mr. Sanders, as he did among voters under 34.

In the open primaries across the South, Mr. Bloomberg had 29 percent support among voters who last participated in a Republican primary, suggesting he was positioned to fare well among the sort of college-educated, affluent voters who flipped from Mitt Romney in 2012 to Hillary Clinton in 2016 and powered the Democratic surge across the Sun Belt.

The few post-South Carolina polls do not necessarily show a collapse in Mr. Bloomberg’s support. If it falters, Mr. Biden could be favored to win in Texas. The extent of advanced voting elsewhere makes it harder to imagine a big Biden upset, but Maine, with one of the oldest Democratic electorates in the country, could surprise.

Another question concerns Ms. Warren, who could easily breach 15 percent of the vote in many Northern states, especially with the help of a few former supporters of Mr. Buttigieg or Ms. Klobuchar. She could also win her home state, Massachusetts. It is hard to see where she could take her campaign from there, but if she could pull off 15 percent in California, it would probably be just enough to block Mr. Sanders from claiming a majority of delegates on Super Tuesday.