Hypothesis No. 4: Voters are behaving tactically. Biden was the only real alternative to Sanders in South Carolina, and he may be the only real alternative going forward.

Tactical voting is something you hear a lot about in multi-party systems like the United Kingdom’s, where voters are trying to find the most viable candidate from a number of similar alternatives (for example, from among the various parties that opposed Brexit). The same dynamics potentially hold in multi-candidate presidential primaries, and we’ve already seen evidence of it. In New Hampshire, voters flocked to Pete Buttigieg and Amy Klobuchar in the closing days of the campaign and away from Biden and Elizabeth Warren. In South Carolina, tactical voting may have worked in Biden’s favor, instead. Biden was fairly clearly the most viable alternative to Sanders, so voters for candidates like Tom Steyer and Buttigieg may have gravitated toward him in the closing days of the campaign.

Degree of concern for Sanders if this hypothesis is true: High. First, if voters are actively looking for alternatives to Sanders — but just can’t settle on which one is best — that can’t be good news for him, and gives some credence to the “lanes” theory of the race in which the moderate vote could eventually consolidate behind one alternative to Sanders. The South Carolina exit poll had Sanders’s favorability rating at just 51 percent, which is some of the stronger evidence for a ceiling on his support so far.