Below are some takeaways based on the results we've seen so far.
1. Joe Biden has a real shot at this
You could have been forgiven for thinking the former vice president was done and dusted after the New Hampshire primary. He finished fourth in Iowa and fifth in the Granite State, behind former South Bend, Indiana, mayor Pete Buttigieg in both and Sen. Amy Klobuchar, D-Minn., in the latter. To the extent this race has a lane for a more moderate, "electability" candidate, he wasn't looking like that candidate. Then came a distant second in Nevada, and it seemed like South Carolina - his firewall state - might suddenly get away from him too.
The opposite happened on Saturday. South Carolina voters responded to Bernie Sanders', I-Vt., third-straight popular-vote win in Nevada by rallying to the guy they want to be the Sanders alternative moving forward. And it gave Biden a real shot at being just that.
It wasn't just the big win; it was also a couple other coinciding dynamics in this race. The first is that Buttigieg and Klobuchar were spectators for the second straight state, with both of those states having been the only really diverse ones in the contest thus far. That's not a great omen for what lay ahead on Super Tuesday or for their electability arguments.
The other is that the other potential Sanders alternative in this race - former New York mayor Mike Bloomberg - is increasingly struggling with poor debate performances and a problematic paper trail. We still have yet to see what his hundreds of millions in spending might draw on Super Tuesday, but the sheen is off, to some degree. And now that Biden has won a state, you have to wonder if that might help him steal some Bloomberg-curious voters who migrated to the former New York mayor when Biden was struggling.
This comes with a caveat, and that's that Biden's win was hardly surprising in the broader context of the race. He dipped a little in the polls after Nevada and looked to be in some trouble, but otherwise his win was in line with his big margins in the state throughout the race. Still, he has to be happy that he was able to make it happen despite his atrocious start this month, and he has to be even happier because of what has happened with some other candidates in the race.
FiveThirtyEight's delegate forecast now has Biden as the only non-Sanders candidate with more than a 1 percent shot at winning a delegate majority. That seems about right.
2. The delegate race begins in earnest Had Sanders somehow won South Carolina, it would be difficult to argue that he hadn't all but sewn up the nomination. And had the result been a little less resounding for Biden, the non-Sanders lane would have remained more muddled and perhaps Sanders would have had a shot at running away with this on Super Tuesday.
The result Saturday, though, suggests we now officially have a delegate race on our hands. Winning the early states matters for momentum and fundraising; starting Tuesday, it will be much more about accumulating delegates.
And Biden's big win Saturday means that delegate race will actually be very close coming out of the first four states, despite Sanders having done much better in the first three (smaller) states. Winning states now will help, but clearing the 15 percent thresholds for delegates will also be hugely important. And competing in every kind of state will also matter. Sanders' opponents are struggling to hit 15 percent in the all-important California primary, for instance, and falling short of that percentage could lead to big shifts.
Also keep in mind: between Tuesday and March 17 two weeks later, more than half the delegates in this race will be handed out. That means the delegate race will come into focus in a hurry.
3. The dropout question
The big question now is who drops out moving forward. Super Tuesday is literally this Tuesday, and among Sanders's opponents there is an increasing premium on consolidating the non-Sanders vote (though it may not be quite as simple as shrinking the field).
Klobuchar seems likely to come under pressure for a few reasons. First, she finished a surprisingly strong third in New Hampshire, but otherwise she has been a nonentity. She was fifth in Iowa, sixth in Nevada and now is very far off the pace in South Carolina. Second, she's the most ideologically similar to Biden, meaning her exit would most apparently accrue to his benefit. And lastly, Buttigieg has a better argument for staying in the race, given he notched a delegate win in Iowa and polls much better than her nationally.
At the same time, given she is far back in the polls, it's not clear how much Biden might gain by her dropping out. And Biden's campaign is reportedly encouraging her to stay in, perhaps in hopes that she prevents Sanders from a significant delegate win in her home state of Minnesota on Tuesday.
There will certainly be questions about whether others should stay in, including Buttigieg. But he's still in the double digits nationwide, and he notched a delegate win in Iowa. What's more, his support spans both wings of the party, so it wouldn't so obviously help Biden if he dropped out.
As for Sanders, his supporters will want Elizabeth Warren, D-Mass., to drop out, and she like Klobuchar is at risk of losing her home state to Sanders on Tuesday. But she has more of a pulse in this race right now, and just Saturday she got the endorsement of the head of the American Federation of Teachers.
4. The Clyburn bump?
The big early exit poll finding on Saturday night was that nearly half of South Carolina voters said that House Majority Whip James Clyburn's, D-S.C., endorsement of Biden this week was important to their vote.
That might have had as much to do with Biden's large margin as anything - why wouldn't his voters approvingly nod to Clyburn's endorsement, after all - but generally people don't admit endorsements actually have an impact on their votes. And Biden's ultimate margin looks like it might be even bigger than the late polling in the race suggested. There were a fairly large number of moderate, black voters who were undecided heading into primary day this week; it seems possible Clyburn helped nudge them to Biden.
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