Castro and former Rep. Beto O’Rourke of Texas are two candidates who debated last month who will not appear on the debate stage for the next round next week. Castro failed to meet the threshold requirements set forth by Democratic National Committee Chairman Tom Perez, while O’Rourke dropped out of the race last month.
Colorado Sen. Michael Bennet, Montana Gov. Steve Bullock, former Maryland Rep. John Delaney and author Marianne Williamson also failed to qualify.
This month, candidates were required to have reached 3 percent support in at least four qualifying national polls since Sept. 13 or 5 percent in two early-nominating state polls since that date, while collecting contributions from at least 165,000 unique donors -- with at least 600 each in a minimum of 20 states.
Some candidates have criticized Perez for what they've viewed as overly stringent requirements. Some argue that the donor emphasis has forced them to spend disproportionately for online fundraising efforts that drain resources they could be using to reach voters in other ways.
Perez says hopefuls have had ample time to demonstrate their supporter, both in polls and through small-dollar contributors, and that any Democrat falling short this far into the campaign almost certainly isn’t positioned to win the nomination or defeat President Trump in November 2020.
Perez has announced even stiffer requirements for a Dec. 19 debate. The polling marks:4 percent in four national polls or 6 percent in two early state polls taken after Oct. 16. The donor threshold: 200,000 unique donors, with at least 800 from each of 20 states.
The top-tier candidates – former Vice President Joe Biden, Massachusetts Sen. Elizabeth Warren, Vermont Sen. Bernie Sanders and South Bend, Ind., Mayor Pete Buttigieg -- are not threatened by those goals. Sens. Kamala Harris of California and Amy Klobuchar of Minnesota have met them as well.
But the higher targets put pressure on several other candidates to broaden their support or risk falling out of any reasonable contention with less than three months to go before the Feb. 3 Iowa caucuses.
Two new candidates also could be vying for December spots.
Former Massachusetts Gov. Deval Patrick launched his campaign Thursday and filed to appear on New Hampshire’s first-in-the-nation primary ballot. Former New York City Mayor Michael Bloomberg is considering a bid as well, already having filed paperwork for some Super Tuesday primaries.
Next Wednesday's debate in Atlanta will be broadcast on MSNBC, 9-11 p.m. EST.
The Associated Press contributed to this report.