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The magic number to win the Democratic Party’s nomination for president is 1,991 pledged delegates.

No candidate has yet reached that number, but following the March 17 primaries, former Vice President Joe Biden became the first to cross the 1,000 pledged delegate mark, and has established a significant lead on his closest rival, Sen. Bernie Sanders. The final candidate remaining in the race, Rep. Tulsi Gabbard, has won only two delegates so far.

Beyond the race’s 3,979 pledged delegates, there are also 771 automatic delegates, otherwise known as “superdelegates.” After a contentious 2016 primary, the Democratic National Committee (DNC) changed its rules around superdelegates so they may now vote on the first ballot to select a nominee at the Democratic National Convention only if a campaign has secured a supermajority of pledged delegates. Otherwise, they weigh in during a second round of voting at the convention.

Candidates are competing for these delegates against a backdrop of complex rules: Delegates are awarded proportionally, and most states and districts require a minimum threshold of 15 percent of the primary or caucus vote to earn pledged delegates. This may make the contest fairer but means it’s harder to secure that magic number in a contested primary until much later in the year.

The Iowa caucuses officially kicked off the delegate race on February 3. But while the early states were all about gaining momentum, the race is ultimately about who can bag the most delegates and get to that magic number. March was a critical month for delegate math: Super Tuesday, March 10, and the three contests on March 17 got us more than 50 percent of the way there.

Now that those contests have concluded, Biden has racked up a delegate lead that will be difficult for Sanders to surmount. But many states have still yet to vote, and the contest could continue on further into the spring and summer, ahead of the Democratic National Convention in July.

Vox has a tracker for the total number of delegates awarded, in partnership with our friends at Decision Desk and the Virginia Center for Politics.

Delegate math, briefly explained

The four earliest states — Iowa, New Hampshire, Nevada, and South Carolina — didn’t contain a lot of delegates, but they did winnow the field. This was especially important this year, as several Democratic candidates were still in the race on Super Tuesday, March 3, the first time a large number of delegates were up for grabs.

Winnowing the field — now down, essentially, to Sanders and Biden — is half the battle. The other half is winning enough of the vote to rack up an insurmountable delegate lead.

Doing this is not just about winning states: It’s also about winning delegates at a congressional district level. Most states award delegates proportionally based on state-level totals as well as delegates in each district. District-level delegates are the voters and local activists who sign up through their state party to act as delegates to the convention — together they make up the vast majority of pledged delegates.

It is critical for candidates to get at least a 15 percent threshold in states or individual congressional districts to qualify for pledged delegates. And a big part of figuring out delegate math means campaigns have to focus on crunching the numbers to target certain districts with demographics that could be favorable for them.

For instance, Sanders focused heavily on states with Latinx voters in March, counting on big wins in California and Nevada to boost his delegate count. That did not quite work out as the senator’s campaign hoped, but Biden’s big bets on Southern states that had large black electorates did pay dividends.

Here’s a breakdown of the delegate categories, both for pledged delegates and automatic delegates:

Pledged delegates

District level: 2,591

At-large: 898

Party leaders and state and local elected officials (PLEO): 490

Total: 3,979

Automatic (“super”) delegates

Democratic National Committee members: 445

Democratic members of Congress: 280

Democratic governors: 24

Party leaders: 22

Total: 771

It’s worth noting the DNC’s biggest rules changes were around superdelegates. That means these 771 superdelegates will largely sit on the sidelines on the first ballot unless there’s already a candidate with a supermajority of pledged delegates.

Correction: An earlier version of this story said it takes 1,990 delegates to win the Democratic nomination. The correct number is 1,991.