In a few hours polls will begin closing in states participating in Super Tuesday, the mega-primary when 14 states vote on the Democrat nominee to meet President Trump in the general election this November. Korey Te Hira explains why Super Tuesday matters, and breaks down the complicated rules that govern which candidate emerges a winner.

The Democratic presidential primary could effectively be over today if front-runner Senator Bernie Sanders amasses a delegate lead so large it would be practically impossible to erase. On the other hand, a drawn-out political death match between two opposing visions for the future of the Democratic party could just be getting started. Which path the primary takes and who President Trump’s general election opponent will be depends on which campaign can best take advantage of the arcane rules that govern how delegates are apportioned on Super Tuesday.

Following former Vice President Joe Biden’s strong win in South Carolina (the final of the four so-called early-voting states), the Democratic nominating race has winnowed out most of the poorer performing candidates just as the race expands from a sequential state-by-state battle to a quasi-national one on Super Tuesday. Today 14 states (and one territory) are voting in the primary, including delegate-rich California and Texas. With more than one third of all delegates awarded on this single day, Super Tuesday is a crucial step on the way to amassing the 1,991 delegates needed to become the Democratic nominee.

Senator Sanders’ core of passionate, progressive supporters and strong performances in the early states makes him the favourite to build a delegate lead today over Biden and former New York City Mayor and billionaire Michael Bloomberg. If he can open up a 200 or so delegate lead, the nomination will be his to lose by virtue of the golden rule of the primary process: It’s hard to get delegate lead, but it’s also hard to catch a delegate leader.

So, why is this the case? The number of delegates offered by each state is roughly based on their population. And in each state, there are two types of delegates awarded: about one-third are awarded at-large (AL) (that is, statewide) and about two-thirds of delegates are awarded by congressional district (CD). In order to be awarded a share of the AL delegates, candidates must clear a 15% threshold in that state. Similarly, candidates must get at least 15% in a congressional district in order to win any CD delegates.

This means there is a strategic imperative to cross the 15% threshold in as many states and districts as possible. This gives a clear advantage to candidates like Sanders who can count on their loyal supporters turning out for them no matter what. The cost of missing out on AL delegates is obvious – California alone is awarding 144 this way, so not getting above 15% statewide would be a major lost opportunity and a gift to other candidates who are viable.

But CD delegates are arguably more important due to roughly two-thirds being awarded this way (271 in California). There are hundreds of districts across the U.S. and on Super Tuesday any given district will have around 2 to 10 CD delegates to award. To build a lead over their opponents, candidates must net CD delegates by winning by big enough margins such that the resulting proportional allocation of delegates means they are entitled to more than other campaigns.

Here’s a quick example: say a medium sized district has five CD delegates to award, and out of field of four candidates only Sanders and Biden clear 15% of all votes cast. In order for Sanders to get a favorable 3-2 split (and net one delegate over Biden) he would need to win a simple majority of the combined vote total he and Biden (that is, only the viable candidates) got in that district. But to really begin separating himself from Biden with a 4-1 delegate split, Sanders would need to win more than 70% of his and Biden’s combined vote. And simply due to the maths, getting favorable delegate splits is even harder in districts with an even number of delegates and when multiple candidates meet the 15% threshold.

So when thinking about delegates, the name of the game is to be viable in as many states/districts as you can and to try to put together landslide performances where possible to begin separating yourself from the pack. Consistently doing this across 50 states, in more than 400 districts, and over many months of a campaign is a daunting challenge. Add in hundreds of millions of dollars in attack ads and a current President laser-focused on re-election and it gets even tougher. But the very fact it is so hard to do is why if anyone can generate a big delegate lead on Super Tuesday, it may be impossible for others to catch them.

You now know why there is such a freak out from establishment Democratic politicians and officials over Sanders’ chances at winning the nomination. He has a loyal, core group of supporters that will likely propel him above 15% in many states and districts. And although former Mayor Pete Buttigieg and Senator Amy Klobuchar have dropped out of the race and endorsed Biden, likely helping push him and possibly even Bloomberg over the viability threshold in more districts and states, the fact that both Biden and Bloomberg are drawing on a similar pool of moderate Democratic voters still makes it more difficult for each to consistently reach 15% of the vote. And even if they do cross the 15% threshold, they may not win by enough to net significant delegates over Sanders.

On the other hand, if Sanders’ lead is kept narrow, then it’s game on. This is especially true with Biden staking a claim to be the main Sanders alternative following his blowout South Carolina win. For those not wanting a Sanders vs. Trump general election to start on Wednesday March 4th, there are three more reasons it mightn’t be all over yet. First, there’s billionaire Bloomberg’s unprecedented war chest of money to spend on ads, field staff, or anything else he could conceivably want between now and the end of the primary, either promoting himself or tearing down Sanders. Until now Bloomberg has been treated like the Night King from Game of Thrones – an ominous and inevitable threat that everyone has yet mostly ignored due to the focus on their immediate political squabbles. He may be about to make his money’s presence felt.

Then there’s one-time front runner Senator Elizabeth Warren, who also appears determined to stay in the race until the convention, accumulating delegates to hopefully use as leverage over whoever the nominee ends up being. And seeing as she shares many voters with Sanders, many of the delegates going to her are lost opportunities for him. But, most importantly, the news media loves a dramatic horse race, with come-from-behind performances and dramatic final stands. Sanders being declared the de-facto nominee Tuesday would deny them months of ongoing juicy speculation and commentary, so they are incentivised to pump up a challenger – and Biden can play the role well.

But forget the spin. Whether Sanders can all-but win the nomination on Super Tuesday – or whether the primary is set for a long slog – all comes down to hard maths and delegate apportionment rules.

Korey Te Hira graduated from the Harvard Kennedy School of Government in 2019 with a Master in Public Policy. He writes a weekly newsletter on US politics.