As Democratic presidential candidates barrel into the fall, they are still waiting for the party to write the rules for the most important events of the campaign.

The candidates are bracing for another round of strict new debate criteria from the Democratic National Committee, which has already halved the list of candidates making the debate stage and could once again shrink it drastically within weeks.

Ten candidates participated in September’s DNC debate and 11 have qualified for the October edition, but the DNC has yet to spell out the thresholds it will use to ration debate participation in November and December with time running short. Even small changes in the criteria could have far-reaching effects: When the DNC set its thresholds at 65,000 donors or 1 percent in polls earlier this year, 20 candidates made the stage.

But merely increasing the polling threshold to 2 percent in four DNC-approved surveys, and doubling the donor threshold, capped the September debate at one night. Even a modest increase for November could spell the end of several campaigns that are just hanging on to the debate stage, and while we know the criteria are likely to keep going up, no one knows exactly how it will affect the 2020 field. And in a nationalized presidential election, the debates have proven to be the most important opportunity for candidates to introduce themselves to large audiences and try to change the direction of their campaigns.

“My only complaint with the DNC’s process is that they haven’t announced what the heck is going on for the November debates yet,” Democratic candidate Andrew Yang told POLITICO, asking whether any reporters knew what the new thresholds would be.

Yang has made every debate so far and is confident of meeting future criteria, and he said “the DNC’s thresholds to make the debates have been incredibly helpful, because then you just know what to aim for."

The next thresholds will shrink the field — but the exact polling and donor criteria for the last debates of 2019 could be the difference between a still-crowded stage and one restricted to just a handful of front-runners. In interviews, DNC Chairman Tom Perez has said that they will “obviously” raise the bar to participate in the debates at the end of the year, because candidates “have to make progress.”

“As we get toward November, December, obviously we will continue to raise the bar of participation, because that’s what we’ve always done,” Perez said earlier this month on “ABC This Week.” “As we get closer and closer, people have to make progress. Two percent, quite frankly, is a very reasonable bar.”

The DNC declined to comment for this story.

Just bumping the polling requirement up to 3 percent could prove to be a significant barrier for some of the 11 candidates who are expected to be in the DNC’s debate in Ohio next month.

“For the candidates at 1 or 2 percent, It is hard to see how they could double that. Not impossible, but tough,” said Peter Brown, a former polling analyst for Quinnipiac University. “If [the threshold goes up], there’s a couple of candidates that will be on the fence.”

Candidates like billionaire Tom Steyer, former Housing and Urban Development Secretary Julián Castro and Sens. Cory Booker and Amy Klobuchar occasionally hit 3 percent in DNC-approved polls, but not reliably.

Moving the requirement higher still, to 5 percent, could imperil all but the top-tier candidates, threatening to leave off candidates like former Rep. Beto O’Rourke or Yang. (No matter what the new thresholds are, the top five candidates — former Vice President Joe Biden, Sens. Elizabeth Warren, Bernie Sanders and Kamala Harris and South Bend, Ind., Mayor Pete Buttigieg will almost assuredly be able to clear them without much trouble.)

It’s also possible the new donor threshold could trip up a candidate or two — though no candidate who hit the polling mark for the September and October debates failed to hit the donor mark.

The candidates who haven’t been able to clear the lower thresholds — like New York Mayor Bill de Blasio or Sen. Michael Bennet — would face even a tougher climb to qualify for future debates after missing out on both September and October. Rep. Tulsi Gabbard (D-Hawaii), among others, has been sharply critical of the process, with her campaign and supporters calling the rules opaque and demanding that a wider variety of polls to be accepted. Gabbard is currently just short of qualifying for the October debate.

“It seems like it is not criteria that is measuring the effectiveness of a future president,” said Jan Bauer, a DNC member from Iowa who supports Montana Gov. Steve Bullock. “The DNC is usurping our roles … That’s been Iowa’s role in the past, and that’s been the beauty of the caucuses. Everyone is given a fair chance.”

Already, missing a debate has contributed to the end of several campaigns. Rep. Eric Swalwell (D-Calif.), who participated in the first primary debate, ended his campaign after it looked like he would be bumped from the second. Sen. Kirsten Gillibrand (D-N.Y.) ended her campaign after a late push to make the September debate fizzled out.

De Blasio has said he’ll likely end his campaign if he does not make the October debate. Qualification closes on Oct. 1.

“There just seems to be too much discussion about qualifying for the debate, somehow that’s become its own primary,” said Philippe Reines, a longtime Hillary Clinton aide. “We’ve added qualifying for debates as a litmus test, no different than fundraising … . This is the closest thing to a weeding out process.” Reines said he would favor an undercard debate.

But the rules are operating as the DNC intended. In interviews, Perez has long said that candidates should remain on the debate stage only if they show progress, and has argued that the debate rules are fair.

“I think it is a good thing, from the party point of view, but of course some of the candidates” don’t agree, said Mark Mellman, a veteran Democratic pollster who is not affiliated with a presidential campaign. “The truth is, most of them would be winnowed in some way, if it wasn’t the debate.”

A recent POLITICO/Morning Consult poll found that a majority of Democratic primary voters believe there were too many candidates on the debate stage.

“Voters are very clear, they want the field to narrow and have fewer choices,” said one staffer for a higher-polling Democratic candidate, who was granted anonymity to speak freely. “They’re overwhelmed by the number of people running. I think that speaks volumes.”