WASHINGTON - Julián Castro, Beto O’Rourke and several other 2020 Democratic hopefuls must generate more support under new thresholds set by the Democratic National Committee for a ticket to a November candidate debate.

In an ongoing effort to winnow the field, the party said Monday that it will require an increase over qualifying thresholds for the September and October debates both in individual donors and polling.

Candidates must receive contributions from at least 165,000 individual donors and score at least 3 percent in four national or approved state polls, or 5 percent in at least two polls in the early-voting states - Iowa, New Hampshire or South Carolina.

The polls must be published between midnight Sept. 13 and a week before the November debate. The date and location have not been announced.

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The new rules are not as stringent as some second-tier campaigns had feared. Nonetheless, Castro is among candidates who could be challenged, along with Minnesota Sen. Amy Klobuchar, New Jersey Sen. Cory Booker, and businessmen Andrew Yang and Tom Steyer.

O’Rourke has reached the donor threshold and his campaign claims one qualifying poll.

Castro, who was the final candidate to qualify for the debate in Houston Sept. 12, last registered 3 percent in a poll early this summer. In September, Castro reached two percent in just one poll out of 12 that were tracked by RealClear Politics.

In a fundraising email to supporters on Friday, Castro responded to speculation that he could be excluded in November by saying: “I can’t believe what I just read. The system has been working against me since day one. Now, they could cut me from the next debates. I need your immediate $5 to ensure I meet the upcoming donor threshold to be on that stage.”

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Castro, O’Rourke and eight other candidates took part in the Houston debate, which drew 14 million viewers, the second-largest audience since the Democratic debates began in June. Candidates who don’t have the opportunity to present themselves can find it difficult to attract donors as well as backing from Democratic voters.

Eleven candidates — including Castro and O’Rourke — have qualified for a debate next month in Ohio by reaching the party’s earlier thresholds of 130,000 donors and four polls of at least 2 percent.