Then he won South Carolina. After each of the first four contests, there were big shifts in the Democratic nominating contest, two of which disadvantaged Biden directly. But this fourth shift allowed him a road back into contention, a shift previewed in late polling and made manifest in the Super Tuesday results.

Remember, this was the Biden campaign whose surprisingly poor performance in Iowa held open the door for former South Bend, Ind., mayor Pete Buttigieg, who ran even with Sen. Bernie Sanders (I-Vt.) in the state. Former New York mayor Mike Bloomberg was pouring money into ads and staff in Super Tuesday states, and Democrats began considering him as a moderate alternative in the race, too. New Hampshire exacerbated Biden’s problems after he finished in fifth.

Right before Nevada, Biden looked as if he would lose South Carolina, too. Then Bloomberg stumbled, faring horribly in the first debate in which he made an appearance. Biden rebounded a bit nationally — but began to spike in South Carolina. The night he won, we created this graph, showing how quickly and dramatically his fortunes in the state shifted.

At that point, the national uptick was more modest. But Biden followed his South Carolina win with big endorsements from his former competitors: Buttigieg, Sen. Amy Klobuchar (D-Minn.) and former Texas congressman Beto O’Rourke (D). Polling showed a massive swing toward Biden even in just the handful of days after the South Carolina contest.

It was hard to know how real that change was. Were Democratic voters really making such a quick, hard shift back toward the guy who led in the polls for most of 2019?

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The answer, it seems, was yes. On Super Tuesday, the media could quickly call Virginia for Biden — a state that saw a three-way tie only a few weeks ago. Biden won North Carolina just as easily. Sanders won his home state of Vermont, but Biden earned enough support to win a delegate there, something that Hillary Clinton was unable to do four years ago.

Preliminary exit polls reinforced that last-minute shift to Biden. In state after state, Biden fared much better among voters who’d only made up their minds in the last few days before voting — that is, since South Carolina. In six of the first seven states to return results on Tuesday, Biden earned more of the late-deciding vote than Sanders.

The only exception was Vermont. But even there, the late-decider vote was split nearly evenly between Sanders and Biden, and Biden earned three times as much support from that group as he did from those who had made up their minds earlier.

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There’s one way in which the late Biden surge might hurt him. In states such as Colorado (which Sanders won) and California (where he held a big lead for weeks), much of the vote was already in before South Carolina. In other words, Sanders banked votes that were essentially immune to Biden’s surge. As we noted earlier on Tuesday, we may see a close California contest by the time polls close on Tuesday that eventually becomes a comfortable Sanders lead as absentee votes are tallied over the coming weeks. This is why candidates focus on absentee votes, of course; those votes are, for most intents, set in stone.

In 2016, Sanders spent much of the last few weeks of the primary contest arguing that he was the candidate who had momentum. His opponent then, Clinton, was simply stagnant, he said, while he was cleaning up. That argument now works against Sanders: His biggest successes on Tuesday night may simply be because his strength was in some places embedded in electoral amber. It’s Biden who has energy and momentum, and it’s Biden who overperformed on Super Tuesday.