Led by Sen. Cory Booker, nine Democratic presidential candidates called on the Democratic National Committee on Saturday to ease qualification thresholds for upcoming debates after complaining the debate stage is becoming less diverse.

In a letter obtained by NBC News, they asked the DNC to use the previous criteria of meeting either the grassroots donor or minimum polling threshold, rather than both.

But the DNC refused to change the qualifications.

“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,” DNC spokesperson Xochitl Hinojosa said in a statement.

“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," she continued. "Our qualification criteria is extremely low and reflects where we are in the race.”

All seven candidates participating in Thursday's debate at Loyola Marymount University in Los Angeles signed on to the request — former Vice President Joe Biden, Sens. Bernie Sanders, Elizabeth Warren and Amy Klobuchar, Mayor Pete Buttigieg, Andrew Yang and Tom Steyer — plus Julián Castro, who also did not qualify for the debate sponsored by PBS NewsHour and Politico.

“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 reads.

“As a result, candidates who have proven both their viability and their commitment to the Democratic Party are being prematurely cut out of the nominating contest before many voters have even tuned in — much less made their decision about whom to support,” it continued.

The candidates who qualified for Thursday's debate threatened this week not to attend because of a labor dispute at Loyola Marymount, where the food workers' union says it's been working without a contract.

The union, Unite Here Local 11, said it would picket the debate, and the candidates said they would not cross that line.

In their letter Saturday, the nine candidates pointed to a lack of diversity on the debate stage.

“...While we know this was an unintended consequence of the DNC’s actions, many of the candidates excluded due to these thresholds are the ones who have helped make this year’s primary field historically diverse,” they said.

In its response, the DNC said that after votes are cast in February, debate criteria “will reflect those contests.”

The letter comes as it appears Booker will not qualify for the upcoming debate, and after Sen. Kamala Harris dropped out of the race, which means a black candidate will not be represented on stage.

It also comes amid a growing concern the field is narrowing to predominantly white candidates, renewing calls for voters to decide which candidates should advance in primaries instead of the DNC.

NBC News confirmed that presidential candidate and former Massachusetts Gov. Deval Patrick, who is black, did not receive the letter nor heard from from the Booker campaign. Rep. Tulsi Gabbard's campaign did not immediately respond to a request for comment.

A rival campaign staffer told NBC News that Booker organized the letter in the last couple of days and had only a couple of candidates sign on until locking down the remainder Saturday morning. The staffer also said the DNC has pushed back on the Booker camp privately, and with the other campaigns, saying it will not revert back to earlier standards.

Campaigning in Iowa Saturday afternoon, Yang — the only candidate of color on next week’s debate stage — said Booker texted him asking him to sign onto the letter.

“Many Americans are looking up at the debate stage and are concerned that the candidates don't represent their interests, their perspective, their point of view,” Yang said. “I'm friends with Cory. If a friend of mine asked me to do something and I think it's positive, of course I will do it.

"So I'm excited for the DNC to consider changes moving forward that I think would be positive for the country and the party.”