Julián Castro warns supporters it'll be "the end of my campaign" if he doesn't qualify for November debate

The Democratic presidential candidate needs to improve his standing in the polls to make the stage.

Democratic presidential candidate Julián Castro greets supporters at a rally at James Garfield High School in East Los Angeles.

Democratic presidential candidate Julián Castro is warning supporters that his campaign will be over if he does not qualify for the November debate.

"I don’t say this lightly: If I don’t make the next debate stage, it will be the end of my campaign," Castro wrote in a fundraising email Thursday.

The email sought donations for ads to help him reach the new polling threshold: 3% in four national polls or 5% in two surveys from the first four early voting states. Castro has already met the donor requirement of 165,000 individuals but hasn't met the polling requirement — or even come close.

Castro, the former U.S. housing secretary and San Antonio mayor, told supporters the polling threshold is "designed to cut candidates like me from the running," arguing it benefits wealthier candidates who can afford to air ads to boost their numbers.

"Secretary Castro has said from the start of this campaign that making the debate stage is critical to his success," Castro spokesman Sawyer Hackett said in a statement after the fundraising email went out. "We are confident he will make it and are counting on the support of grassroots donors to get us there."

Although fundraising email solicitations are known for their dramatic language, Castro is at real risk of getting left out of the November debate. While he has the donor count, he regularly registers under 3% — even 2% — in national and early voting state polls.

The other Texan in the race, Beto O'Rourke, also is not a shoo-in for the November debate, though he appears to be in a better position than Castro. He blew past the 165,000-donor mark long ago, and while he also consistently polls in the low single digits, he usually comes in a point or two above Castro.

Candidates have until seven days before the November debate to satisfy the polling requirement. The date and location of the debate have not yet been disclosed.

Both have already qualified for the next debate, which is in mid-October in Ohio.