Castro missing the debate stage comes at a trying time for the Texan’s campaign. Last week, POLITICO reported that he was laying off his staff in New Hampshire and South Carolina, with the intent to focus his now long-shot bid on Iowa and Nevada.

Castro is not the only active candidate who missed out on the debate stage. Michael Bennet, Steve Bullock, John Delaney and Marianne Williamson all participated in earlier debates but will not qualify for the November debate. Joe Sestak and Wayne Messam have never qualified for a debate and will not be on stage next week, either.

To qualify for the November debate, candidates needed to hit 3 percent in four DNC-approved polls (or 5 percent in two DNC-approved polls conducted in the early states) and receive donations from 165,000 unique contributors.

Castro’s campaign said he cleared the donor threshold, but in the more than 30 polls released in the qualifying period, Castro did not hit 3 percent once.

The Castro campaign did not respond to a request for comment.

Missing a debate has been fatal to other presidential candidates who have since ended their presidential bid. Sen. Kirsten Gillibrand (D-N.Y.) made a big bet on making the debate stage, and dropped out after it became clear she would miss the September debate.

Castro, the only Latino in the race, has been on the vanguard of the primary field on several issues. He was the first to release a substantive plan on immigration, and was the first in the field to endorse launching impeachment proceedings against President Donald Trump. But Castro has never particularly caught on with voters in public polling.

He also notably confronted Biden in the September debate, attacking the former vice president with an apparent reference to his age, while needling Biden for continuing to invoke former President Barack Obama.

“Every time something good about Barack Obama comes up, he says, 'Oh, I was there. I was there. I was there. That's me, too.’ And then every time somebody questions part of the administration that we were both part of, he says, ‘Well, that was the president,’” Castro said on stage in September.

As Castro has slipped toward the back of the pack, he has become more willing to criticize the Democratic nominating process. In particular, he has taken aim at the role Iowa and New Hampshire — two states where the electorates are overwhelmingly white, even in Democratic primaries — play in winnowing the field and ultimately determining the nominee.

“I actually believe we do need to change the order of the states, because I don’t believe that that we’re the same country we were in 1972," Castro said on MSNBC on Sunday. “What I really appreciate about Iowans and the folks in New Hampshire is they take this process very seriously. … [But] at the same time, demographically, it is not reflective of the United States as a whole, certainly not reflective of the Democratic Party. And I believe that other states should have their chance.”

Castro has used the specter of missing debates to fundraise from his supporters, telling them he still had a shot of making the debate stage. But in reality, he has not been close to qualifying.

“I only have 24 hours to secure my spot in the next debate -- and as of 6pm, it looks like I might not make it,” Castro wrote in a fundraising email sent early Wednesday morning that was archived by the Defending Democracy Together Institute. “Together we’ve beat the odds time and time again -- but right now, I need you more than ever. Will you rush $5 before midnight so I can fund my final 24-hour push and make the cut before time runs out?”

The road for Castro — and the other candidates who will miss the November debate — only gets steeper for future debates. To qualify for the next debate in December, which is hosted by POLITICO and PBS Newshour on Dec. 19, candidates need to hit four percent in four polls (or six percent in two early-state polls) and have 200,000 donors.

Six candidates have already qualified for the POLITICO/PBS debate in December, according to POLITICO’s tracking (no candidate’s qualification is official until confirmed by the DNC): Biden, Buttigieg, Harris, Klobuchar, Sanders and Warren. Gabbard, Steyer and Yang are currently short of at least one of the thresholds but are within striking distance of qualifying. Other candidates have nearly a month until qualification closes on Dec. 12.