2020 frontrunners back Cory Booker's call to ease DNC debate rules

The big picture: NBC notes the letter is signed by all seven candidates who've qualified for Thursday's Los Angeles debate: former Vice President Joe Biden; Mayor Pete Buttigieg; Sens. Bernie Sanders, Elizabeth Warren and Amy Klobuchar; entrepreneur Andrew Yang and billionaire Tom Steyer.

• Former Housing secretary Julián Castro, who did not qualify for the debate, also signed the letter.

What they're saying: Axios has contacted the DNC for comment. The committee told Politico, "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. Our qualification criteria is extremely low and reflects where we are in the race. Once voting starts in February, our criteria will reflect those contests, which is more than appropriate.

"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."

Go deeper:

• Who has qualified for the December Democratic debate
• Democrats call out lack of diversity in 2020 race after Harris' exit
• DNC raises qualifying criteria for November debates
• 5 takeaways from the November Democratic debate