After more than a year of anguish, electability navel-gazing, and 10 contentious debates, the most important day on the Democrats' nominating calendar has finally arrived. Fully one-third of all pledged delegates to the 2020 Democratic National Convention will be chosen Tuesday. Until this year, there was a heavy Southern bias to Super Tuesday, which can be traced back to the Democratic Party's desire to produce moderate nominees in the late '80s when that region of the country was slipping away from them. But with California moving its primary way up and Super Tuesday staples like Georgia bumped back in the schedule, this year's edition is likely to look very different than it did in the past.

Vermont Sen. Bernie Sanders remains the favorite to win the nomination, but more than perhaps any candidate in the modern nominating system, he needs to secure a pledged delegate majority to ensure his victory. Party elites remain deeply hostile to him, and older Democrats are terrified that he will lead the whole party to the wrong side of the Alamo shootout in November. If he doesn't get a majority, look for months of intrigue as candidates vie for support on a second ballot at the convention, and as the hundreds of so-called superdelegates, barred from voting on the initial tally, are suddenly the kingmakers. While it would probably be suicidal for the party to deny Sanders the nomination if he has a plurality rather than a majority of pledged delegates, the smaller that lead is, in both raw and percentage terms, the more likely it is that the effort to make someone else the nominee will be serious. And by the end of this evening, we will have a much better idea of how likely such a nightmare scenario is.

To further complicate matters, two significant candidates bailed on the race at the last possible minute before Super Tuesday, meaning that we should expect much more significant variation from existing polling than usual. Former South Bend, Indiana, Mayor Pete Buttigieg dropped out of the race unexpectedly on Sunday, followed by Minnesota Sen. Amy Klobuchar Monday. If polls are to be believed, Buttigieg's support will be spread fairly evenly among the remaining candidates, but of course, millions of people, particularly in California, voted early and can't change their minds now. That means that Buttigieg's political corpse will still rack up some votes, although probably not any further delegates. Klobuchar's voters, on the one hand, are likely to disproportionately go to Biden, especially since she endorsed him, perhaps giving him an extra 2 or 3 percent in many states where she was actively campaigning. On the other hand, her decision almost certainly means that Sanders will carry barely-polled Minnesota, where Biden was mired in the single digits, unless she has so much pull that she can throw nearly all over her voters to the former vice president.

No one knows exactly what is going to happen tonight. There are states with precisely zero polls, including Tennessee and Alabama. And many others, including Maine, haven't been surveyed in weeks, or have only a single data point to extrapolate from. But there are probably only three plausible outcomes given what we do know.

1. Sanders wins convincingly enough that no one else has a plausible path to the nomination

The best case for Sanders tonight involves a blowout victory in California, with Biden failing to meet the viability threshold statewide and in enough congressional districts to leave him with just a handful of delegates. The ideal Sanders scenario here is that several candidates — Biden, Warren, and Bloomberg — reach the high single or low double digits but get effectively shut out of delegates. Sanders also needs to beat Biden in Texas, and if he does so by double digits he could really open up an enormous lead. But margins in some states that have hardly been talked about or polled are also important. Biden is likely to carry Alabama and Tennessee, where black voters make up a large percentage of the electorate. But if Sanders can pull out surprise wins in states like Arkansas and Oklahoma, which have large pools of the kind of non-college educated white Democrats that Sanders does so well with, he can really pad his margins. Sanders won Oklahoma in 2016, and it wouldn't be a surprise if he did so again this year. It would also help if he beat expectations in the Deep South, even if he doesn't notch any wins.

Let's say that Sanders also comes out on top in Maine and Massachusetts. The latter would loom particularly large as it would cut the heart out of Warren's campaign. And while her camp has signaled that they are in it for the long haul, this is precisely the kind of happy talk that becomes irrelevant once the votes are counted. Losing her home state would leave Warren with zero wins. And a victory of this magnitude for Sanders would leave Biden hundreds of delegates behind and would effectively end the race for the Democratic nomination. Remember, the Democrats' proportional delegate allocation rules mean that Biden would need not just to win more states than Sanders moving forward, but to win them so decisively that he can actually catch up. And it's hard to see where that's going to happen. After Super Tuesday, there are a limited number of states, among them Georgia, Florida, and Louisiana, where Biden looks to have an edge. Even in the Acela Corridor states of Maryland, New Jersey, New York, and Connecticut, Sanders is likely to run strong enough to keep the delegate totals close, even if he doesn't win.

Even if Sanders exceeds all expectations, Biden is likely to nab a few wins and stick around. But results like these would probably drive Warren and Bloomberg out and leave us with a Sanders-Biden race that would be all but unwinnable for the moderate.

Sanders wins: Okla., Ark., Colo., Calif., Texas, N.C., Mass., Vt., Maine, Minn., Utah

Biden wins: Ala., Tenn.

Bloomberg wins: Va.

Other candidates: none

Sanders delegate lead: 200-300

2. Late movement in the polls after South Carolina allows Biden to fight Sanders to a Super Tuesday draw

What if Biden capitalizes on his huge Saturday win in South Carolina and fights Sanders to a draw. The person most in the way of this Biden renaissance is Bloomberg, who is drawing double-digit support in important states like Virginia and North Carolina. But the former New York City mayor's disastrous debate performances have taken the new car sheen off of his candidacy and exposed him as being well outside the mainstream of the Democratic Party. His meteoric rise in the polls — he doubled his average in national polls during the month of February — has stalled. And because there have been so few polls of Super Tuesday states, we simply don't know whether the bottom has dropped out of his operation or whether he stands to rack up 100+ delegates. For Biden to really come close to Sanders though, Bloomberg's numbers need to be much more mirage than reality.

One key to the this scenario is for Biden to run a strong second in California. There's really no doubt about what will happen in the most heavily polled Super Tuesday state: Sanders will win. But if Biden can run about 20-25 percent statewide, he'll be viable in most districts and Sanders might walk away with a more narrow plurality of delegates. The polls in Texas are also close. If Biden can ride his Palmetto State momentum to a victory there, no matter how narrow, it will help offset the numbers in California and give him an even stronger argument as the most electable candidate. And if he can also sweep the South — Arkansas, Alabama, Tennessee, North Carolina, and Virginia — he could be within 100 or so delegates of Sanders. Biden can also benefit from Elizabeth Warren winning her home state of Massachusetts, which would eat into Sanders' aggregate delegate tally. If you squint, you can see a best-case scenario for Biden in which he basically pulls even with Sanders in delegates.

Sanders wins: Calif., Texas, Colo., Vt., Minn., Utah

Biden wins: Ala., Ark., Okla., Tenn., Va., N.C.

Warren wins: Mass.

Bloomberg wins: none

Sanders delegate lead: 0-150

3. Sanders hits his ceiling, five different candidates win at least one state, and the delegate totals are so fractured that a contested convention looks likely

Unless the polling is truly, dramatically, and epically wrong, Sanders has California in the bag. But the margins are critical. California is Bernie's best bet for putting this whole thing to bed, and if he only wins there narrowly, he could be in for a long, indecisive slog. There are also plenty of states that have been pretty lightly polled that could prove decisive in splitting delegates so many ways that even Sanders will struggle to put together a majority by June.

The critical player here is Bloomberg, who polls best in states like Virginia and North Carolina. If late-deciding voters gravitate towards him rather than Biden, he could win two or more states outright and drag Biden's numbers down even in states that the former vice president wins, such as Alabama. Bloomberg has poured money into states like Tennessee and Arkansas, which have received almost no attention based on the assumption that Biden will win them going away. But what if they're wrong, and Bloomberg's billions have the desired effect? What if Klobuchar wins her home state even after dropping out? Don't laugh — Howard Dean did it in 2004. What if Sanders runs so low in some of the underpolled states that he misses out on large numbers of delegates?

A five-way split would still almost certainly leave Sanders in the lead unless someone else wins California. But a 50 delegate advantage with 40 percent of the overall delegates looks much different than a 50 delegate lead with 35 percent of the overall delegates. Sanders could need to win an unrealistic percentage of the remaining delegates to get to a majority. And with a brokered convention now a real possibility, Bloomberg, Biden, and Warren all might hang around so they can make the argument to delegates that they stayed and fought and deserve the nomination even if they don't have the lead.

Sanders wins: Calif., Texas, Colo., Vt., Maine, Utah

Biden wins: Ala., Ark., Okla.

Bloomberg wins: Va., N.C., Tenn.

Warren wins: Mass.

Klobuchar wins: Minn.

Sanders delegate lead: 50-150

The guess here is that scenario #2 is the most likely outcome on Tuesday. Biden is on the rise, and Sanders seemingly has been unable to capitalize on his early victories and consolidate the support of factions initially inclined to support someone else. That could, ironically, put Sanders in roughly the position Hillary Clinton was in after Super Tuesday 2016, when she led him by about 200 pledged delegates. Just as Sanders was unable to overcome that disadvantage despite a long rally in March and April, Biden may find that even a 100-delegate lead for Sanders will be difficult to overcome. And with the field effectively winnowed down to two candidates, it becomes far likelier that Sanders will eventually reach a pledged delegate majority or get so close that it becomes a moot point. The progressive winning a narrow and hotly contested race over the moderate would be like a mirror image of 2016, and would indicate that indeed, the party has shifted just enough to the left to make Sanders its standard bearer.

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