And then, late Wednesday, a twist: A count of paper ballots that had not been totaled on primary night pushed Melinda Katz — the Queens borough president and establishment favorite — ahead, by the barely-there margin of 20 votes.

The news thrust the borough, as well as the broader New York political world, into chaos. It threw a major victory for the left wing of the Democratic Party into doubt, and it inspired immediate recriminations about traditional party forces.

But the race isn’t over yet, as the vote now goes to a manual recount, automatically triggered by the tiny margin. Cabán’s team has promised to fight for every vote, in what could turn into a protracted, expensive legal battle.

Here’s what you need to know.

What exactly happened?

Cabán had declared victory on primary night with a lead of about 1,100 votes, or 1.3 percentage points.

Her team acknowledged at the time that about 3,400 absentee ballots still had to be counted. But they said they felt comfortable claiming victory because the paper ballots would presumably be split among all seven candidates in the crowded field, making it hard for Katz to come up with enough votes to close the gap.

Now, it appears she did: Katz won 1,901 of the paper ballots counted on Wednesday, compared with Cabán’s 751. That flipped last week’s result to put Katz just 20 votes over.

There’s no question that the outcome was a shock, even from a purely numerical perspective. (Though Katz had refused to concede on primary night, pointing to the outstanding paper ballots.)

Katz had pulled ahead by winning about double the percentage of absentee ballots that she had of regular ballots — a result that Martin Connor, a former state senator who now works as an election lawyer, had thought highly improbable.

“But it’s not the first time I’ve been fooled just by doing a rough mathematical calculation,” said Connor, who did not work for either candidate. “The voters do what the voters do.”

How could this happen?

The Katz campaign says it ran a robust get-out-the-vote effort among absentee voters.

The team sent campaign mail to the roughly 10,000 people who were eligible to vote absentee in the June 25 primary election, then followed up with phone calls, according to Matthew Rey, a partner at Red Horse Strategies, a consulting firm that worked for Katz.

Absentee voters tend to skew older — a constituency that likely would have been more familiar and more ideologically aligned with Katz, said Bruce Gyory, an adjunct professor of political science at the University at Albany. So it made sense that the paper ballots would tilt overwhelmingly in Katz’s favor, rather than breaking down in roughly the same proportions as the primary night totals, he said.

Much of Cabán’s support, by contrast, came from just two Assembly districts, in gentrifying parts of Queens.

“Her coalition was broad, and that was true for absentee votes and for Election Day votes,” Rey said of Katz, his client.

“Our opponent’s coalition was very narrow and focused on one constituency, and the weakness of that sort of coalition is something that became clearer and clearer yesterday,” he added.

Could Cabán still win?

Cabán’s supporters, in casting doubt on Wednesday’s flip, have pointed to another type of potentially game-changing ballots besides the absentee ones: affidavit ballots.

Affidavit ballots, also known as provisional ballots, are given to voters who show up at the polls but whose names do not appear in the voter rolls. Those people still get to vote if they sign an affidavit affirming that they are eligible to do so. The Board of Elections later verifies those claims, throwing out any ballots it deems invalid.

Of the roughly 2,800 affidavit ballots cast in the Queens Democratic primary, more than 80% were thrown out by elections officials. Just 487 were counted.

Cabán’s supporters have suggested that those disqualified ballots hold the key to her victory, and they’ve promised to fight to get them revalidated.

There are several reasons an affidavit ballot could be disqualified.

New York has closed primaries, meaning that only registered Democrats can vote in their party’s primaries. Someone registered with the Green Party or Working Families Party — or not affiliated with any party — would not be allowed to cast a ballot, even if they considered themselves a lifelong liberal.

In addition, New York’s arcane voting laws require changes in party affiliation to be submitted to the Board of Elections at least six months before the election date — far longer than in any other state. (A bill making it easier to change parties passed the Legislature last month but has not been delivered to the governor.) Some affidavit ballots could have been disqualified if the voter registered as a Democrat too late.

Other issues — showing up at the wrong polling site, or not being registered at all — can also get a ballot tossed.

It’s possible those rules could have affected many affidavit voters who supported Cabán, as her supporters tended to be newer to the political process.

Jerry H. Goldfeder, an election lawyer working for Cabán, acknowledged that it was “not unusual to invalidate a lot of affidavit ballots because people are really not eligible to vote in a particular primary.

“Unfortunately, that happens all the time,” he said.

But, Goldfeder continued, “this is an inordinately large number. And we’ve already discovered a large number of those ballots where they shouldn’t have been invalidated.”

Any other reason for the flip?

Almost immediately after the news broke of Katz’s breakthrough, people began speculating about more nefarious explanations.

In particular, Cabán’s supporters have accused the Queens County Democratic Party, whose leaders supported Katz, of engineering the disqualification of so many affidavit ballots. They pointed out that the commissioners of the Board of Elections are selected by the county party leaders, before being approved by the City Council.

Shaun King, a prominent progressive activist, wrote on Twitter that the Queens County Democratic machine had tried to “steal” the election for Katz. State Sens. Alessandra Biaggi and Julia Salazar, who won their seats last year by challenging incumbent Democrats from the left, also blamed Katz or the county commissioner for Cabán falling behind.

Cabán did not outright accuse party officials of conspiring against her. But she invoked her outsider status, as she has done throughout the race.

“Our campaign, and all of Queens, is up against a party machine that has ruled local politics and suppressed democracy for decades,” she said in a statement on Thursday. “We are still fighting to make sure every valid ballot is counted. We are confident that if that happens, we will be victorious.”

Rey, from Katz’s campaign, said legislators and activists who were criticizing the absentee and affidavit ballot procedures, or even suggesting that they had been manipulated by Katz’s supporters, were undermining an important democratic process.

“It’s irresponsible, reckless, and completely wrongheaded to claim any sort of malfeasance for a practice that has been completely liberalized over the past decade,” he said.

When we will we have the final result?

It’s hard to know.

On Friday, lawyers for both candidates will argue before the Board of Elections why certain affidavit ballots should or shouldn’t be counted.

The recount is expected to kick off on Monday. That means elections officials will find a place to put 18 tables — one for each Assembly district in Queens — where two staffers, one Democratic and one Republican, will tally each and every ballot that was cast: absentee, affidavit or regular.

That process could turn up untold numbers of new votes. The city’s ballot machines count only ballots with perfectly filled-in bubbles, but in a manual recount, ballots that were improperly filled out — a check mark next to a candidate’s name, for example, or a circle around it — can be counted.

“It could take two days, it could take five days,” Goldfeder said. “It depends on how many disputes there are.”

And by all indications, we can expect disputes.

What does it all mean?

Cabán’s apparent win last week was hailed as a major triumph for the far left wing of the Democratic Party, one that showed that the progressive energy that fueled Rep. Alexandria Ocasio-Cortez’s win in one congressional district could translate to a larger, more diverse arena.

If Katz emerges victorious, it will suggest that more traditional political paths — inching up the ranks of local politics, earning endorsements from unions and other elected officials — still retain much of their power.

And it might highlight the need for progressive candidates to make more inroads with older, religious and minority voters, Gyory, the professor, said.

But that would still not negate the forces that put Cabán within a hair’s breadth of victory, he added.

“Whichever way it ends up, it’s tough to make this a case to show that the left is ascendant,” he said. “It’s also tough to say that there’s not a strong progressive pulse.”

This article originally appeared in The New York Times.