Booker later dug in on Twitter, reflecting on how one of the most initially diverse fields in history has dwindled: “We're spiraling toward a debate stage without a single person of color.”

Former Housing and Urban Development Secretary Julián Castro echoed Booker’s sentiments, arguing coverage of Harris’ campaign was “grossly unfair and unfortunate.” Pointing to stories by The Washington Post, POLITICO and The New York Times, Castro decried the media for pushing a “flawed formula” of electability that subjects women and candidates of color to a double standard.

Submit a question for the next debate We're hosting the sixth Democratic debate on Dec. 19, and we want your input. What would you ask the candidates? Tell us by Dec. 13.

“Our party’s diversity is our strength, and it’s a shame that we’re headed for a December debate without a single person of color,” Castro later tweeted.

Aside from Booker and Castro, the other non-white candidates in the race — entrepreneur Yang, Gabbard and former Massachusetts Gov. Deval Patrick — have not hit the DNC's polling and donor thresholds. Four white men and two white women have qualified: Former Vice President Joe Biden, South Bend Mayor Pete Buttigieg, billionaire Tom Steyer, and Sens. Bernie Sanders, Elizabeth Warren, and Amy Klobuchar.

Andrew Gillum, who ran unsuccessfully for Florida governor last year, tweeted links to both Booker and Castro's campaign contribution pages, urging people to help them make the debate.

"I'm donating to both @JulianCastro & @CoryBooker — we need them on that debate stage," the African-American former mayor of Tallahassee tweeted Wednesday. "Our diversity is our strength & our debates must reflect that fully."

While Democrats expressed concern about the narrowing field's dwindling diversity, some Republicans took to Twitter to mock that fear. Wyoming Rep. Liz Cheney quoted a tweet listing the all-white candidates on the December stage with the quip: “You forgot about Pocahontas,” invoking President Donald Trump's nickname for Warren.

Warren apologized earlier this year for identifying as Native American earlier in her career.

