With the end of February comes the end of the early delegate-selection events, which are dominated by spin and stories and knocking out candidates. Now we come to the events, beginning with Super Tuesday today, in which what matters more than anything is the delegate count.

The combination of landslide wins for Bernie Sanders in Nevada and Joe Biden in South Carolina — in each case with the other finishing a distant but solid second — has finally winnowed the field. Pete Buttigieg and Tom Steyer dropped out of the race this weekend.

That leaves six major contenders — five if we don’t count Tulsi Gabbard, who has no delegates and no prospects of winning any.

Amy Klobuchar may win her home state of Minnesota today, but after that (or perhaps even earlier) she’s likely to join Buttigieg and Steyer. Michael Bloomberg might win some delegates on Super Tuesday, but there’s a good chance he’ll finish behind Biden in each state that votes, and unless the polls are entirely wrong, he’ll have no path forward from there.

Elizabeth Warren, like Bloomberg, stands to win some delegates this week, but unless her strong campaign organisation beats the polls significantly, she, too, will have no path to winning after today.

No, at this point it’s even more clear than it was after Nevada: only Sanders or Biden has a realistic chance of winning the nomination, barring something extraordinary.

So how did Biden do it?

One theory is that it was just demographics: once Biden escaped from the largely white electorates in Iowa and New Hampshire, he was going to improve. The problem with that is the polling, which showed Biden collapsing in South Carolina for much of February. That suggests that there was nothing inevitable about Biden’s strong showing.

Another theory is that House Democratic Whip James Clyburn’s endorsement made the difference. I’m sure that helped, but the endorsement was on Wednesday, and Biden’s polling had already started surging before that.

Which gets to the theory I suspect is correct: the results in Nevada provided the structure for the nomination contest that everyone had been waiting for all along. By finishing second, Biden became the Sanders alternative — and by winning big, Sanders made it necessary for those who didn’t want him to be the nominee to settle for whatever alternative was available.

That theory is consistent with the polling, which shows Biden improving immediately after Nevada. If it’s correct, then we can think of Clyburn’s endorsement as an effect of Nevada as much as it may have been a cause of what happened in South Carolina.

This is consistent with reporting from on-the-trail journalists such as the Washington Post’s David Weigel. It’s also consistent with what the political science literature says about most voters in primaries: they often don’t focus on the contest until right at the end, and they don’t really want to consider all of the candidates who have been running — they just want some signal about which ones are the serious contenders.

I can’t prove that Warren, Buttigieg or even Klobuchar would have surged in South Carolina had one of them finished second in Nevada. Each of them, after all, has had difficulties winning the support of black voters.

And I’m not saying that they would have had the exact same win as Biden did. But I do think a lot of voters, party actors and media folks were poised to treat whoever finished second in Nevada as the natural alternative to Sanders, and the others as also-rans — with predictable results.

What’s harder to guess is what’s next. FiveThirtyEight’s Nathaniel Rakich is correct: the situation may well be moving too fast for the polls to measure before votes are cast today (and remember that a lot of Super Tuesday voters have already voted early — and that counting the votes, especially in delegate-rich California, will take a long time).

Indeed, when Rakich wrote that on Sunday at noon, the FiveThirtyEight model projected Sanders would beat Biden by 145 delegates on Super Tuesday, including an 84-delegate margin in California; as I write this, that California gap has closed to a projected 63-delegate margin. And that’s without any polls to measure the effects of Buttigieg dropping out, and almost nothing from after the results were announced in South Carolina.

We simply don’t know where Buttigieg voters (or even the handful of Steyer voters) might go, or what the effects of Biden’s big win will be. Will it further consolidate voters who don’t want Sanders to win behind Biden?

Could it actually help the Vermont senator, by pushing voters who might have another first-choice candidate but prefer Sanders over Biden to switch to Sanders?

Or could some Sanders bandwagon voters abandon him for Warren or someone else? We’ll get a few more polls, but mostly we’ll just have to wait and see.

I will say this: that noon Sunday forecast for Super Tuesday predicted 540 delegates for Sanders, 395 for Biden, and a whopping 410 for the other candidates. That’s why the model expects a contested convention, or at least no one reaching a majority by the end of the primaries and caucuses. It’s possible that, as Nate Silver explains, Buttigieg dropping out could actually increase the number of delegates going to others by helping Bloomberg, Warren and Klobuchar reach the magic 15 per cent share of the vote needed to receive delegates in more states and congressional districts.

But I strongly suspect that Sanders and Biden will in fact combine for a larger share of the Super Tuesday delegates — and then the overwhelming share of the delegates selected after that, as the other candidates drop out or just fade from relevance.

If I’m wrong and the FiveThirtyEight model is correct, however, then we are headed for a contested convention, chaos and a potential disaster for the Democrats. So really there’s a lot on the line today, and a lot of very plausible results.

• Jonathan Bernstein is a Bloomberg Opinion columnist covering politics and policy. Disclaimer: Michael Bloomberg is the founder and majority owner of Bloomberg LP, the parent company of Bloomberg News