As the rest of the field slowly consolidates, Sanders could gradually increase his vote share just enough to win a narrow majority. Even if the moderate lane consolidated to just one or two alternatives later on in the race — say, at some point in March or April — Sanders would still be in a pretty decent position. He would probably have a head start on the competition by having won a lot of delegates on Super Tuesday and in the first four states while the rest of the field sorted itself out. Contests up to and including Super Tuesday account for 38 percent of all pledged delegates, so this matters a lot.
Also, even if there is some upward resistance to Sanders’s numbers — more than there might be for the average candidate — it isn’t likely to be absolute resistance. Case in point: Sanders improved his support from 15 percent in national polls for much of last year to the low-to-mid 20s now. Without those gains, Sanders might be in the fairly difficult position that Warren now finds herself in, following third- or fourth-place finishes in Iowa and New Hampshire.
Making slow-but-steady polling gains was roughly the path that Trump followed in 2016 to win the Republcian nomination, too. True, Trump had one major advantage that Sanders didn’t: the presence of winner-take-all states, especially later on in the race. (All Democratic states use proportional delegate allocation above the 15 percent threshold.) Still, Trump gained ground later in the race once Republican voters realized that they faced a choice between Trump and a contested convention (which might nonetheless have resulted in his nomination). Democratic voters might act similarly. Maybe a voter would prefer Buttigieg to Sanders in the abstract, but if a Buttigieg win would require a contested convention, while a Sanders win would not, she might feel differently.