Washington (CNN) Nine Democratic presidential candidates signed a letter to Democratic National Committee leaders on Saturday urging the DNC to consider changing the debate qualifications for the January and February presidential primary debates.

The candidates, seven of whom have qualified for next week's debate in Los Angeles, argue in the letter that the DNC's increasing thresholds have resulted in candidates being "prematurely cut out of the nominating contest." The candidates being excluded from the debate, the letter states, "are the ones who have helped make this year's primary field historically diverse."

Entrepreneur Andrew Yang is the only person of color who qualified for next week's debate. New Jersey Sen. Cory Booker and former Housing and Urban Development Secretary Julián Castro both did not qualify and have been outspoken about the lack of diversity in the 2020 race following the exit of California Sen. Kamala Harris earlier this month.

"The escalating thresholds over the past few months have unnecessarily and artificially narrowed what started as the strongest and most diverse Democratic field in history before voters have had a chance to be heard," the letter states.

The letter calls for the DNC to increase access to the stage by "returning to the previous criteria that allowed candidates to qualify to participate either via meeting a minimum polling threshold or meeting a number of grassroots donors to demonstrate broad-based support."

A spokeswoman for the DNC, Xochitl Hinojosa, said in a statement that the DNC "has led a fair and transparent process and even told campaigns almost a year ago that the qualification criteria would go up later in the year- not one campaign objected."

"The DNC will not change the threshold for any one candidate and will not revert back to two consecutive nights with more than a dozen candidates," Hinojosa said.

Former Vice President Joe Biden, South Bend, Indiana, Mayor Pete Buttigieg, Minnesota Sen. Amy Klobuchar, Vermont Sen. Bernie Sanders, businessman Tom Steyer and Massachusetts Sen. Elizabeth Warren have qualified to join Yang on the December debate stage, and all signed the letter.

Campaigns for Booker, Castro and Hawaii Rep. Tulsi Gabbard said they hit the fundraising threshold, but did not have the required number of polls to make the stage. Gabbard said she would not attend the debate even if she did qualify. Gabbard did not sign the letter, and did not immediately respond to CNN's request for comment.

A DNC official told CNN that despite signing the letter, a number of Democratic campaigns privately expressed their frustration to the DNC about the large debate stage and asked the DNC to "hold the line" on their thresholds.

Hinojosa called the current qualification criteria "extremely low" and reflective of this point in the primary process.

"Once voting starts in February, our criteria will reflect those contests, which is more than appropriate," Hinojosa said. "We're proud to have given candidates so many opportunities to get their message across, and will continue to have fair criteria that reflects each point in the race."

The DNC has been raising the thresholds for the contests, slowly shrinking the field of Democrats on the high-profile debate stages.

In order to qualify for next week's debate, Democratic presidential candidates needed to receive 4% in at least four national or early state polls that met the DNC's criteria or 6% in two early state polls. Candidates also needed to receive donations from at least 200,000 unique donors, with a minimum of 800 from at least 20 different states. Candidates had until Thursday to meet the fundraising and polling thresholds.