IOWA CITY — Nine Democratic presidential candidates have called on the Democratic National Committee to relax its debate standards next year, allowing some lower-polling rivals onto the 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,” the candidates wrote. “Frankly, that unintended result does not live up to the values of our Democratic Party and it does not serve the best interest of Democratic voters.”

The letter was signed by all seven Democrats who qualified for next week's debate in Los Angeles: former vice president Joe Biden; Sen. Bernie Sanders (Vt.), Sen. Elizabeth Warren (Mass.), South Bend, Ind. Mayor Pete Buttigieg; Sen. Amy Klobuchar (Minn.), investor Tom Steyer; and businessman Andrew Yang.

It was also signed by Sen. Cory Booker (D-N.J.), who organized his fellow candidates, as well as former housing and urban development secretary Julián Castro, who has not appeared in a debate since October. Castro has argued that the debate standards and the structure of the primaries — in which two of the country’s whitest states vote first — were leading to a less diverse Democratic contest.

“Cory sent me a text message,” Yang said after a town hall here, explaining why he signed the letter. “I’m friends with Cory. If a friend of mine asks me to do something and I think it’s positive, of course I’ll do it. So, I’m excited for the DNC to consider changes moving forward that I think would be positive for the party and the country.”

The letter was obtained Saturday afternoon by The Washington Post after BuzzFeed News reported on its contents. In a statement, DNC communications director Xochitl Hinojosa defended the “fair and transparent process” announced by the party in early 2019, noting that campaigns had been advised from the outset that the standards would increase after the first debates.

“Not one campaign objected,” Hinojosa said. “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.”

Those standards, which arranged candidates for the first debates this year, allowed Democrats onstage if they attracted 65,000 unique donations or hit 1 percent in at least three polls. In the end, 21 Democrats met that standard, with former senator Mike Gravel (D-Alaska) missing the cut only because the debates were capped at a total of 20 candidates over two nights.

The DNC steadily raised the bar after July’s debates in Detroit, requiring candidates to hit both a higher polling standard and higher donation number. No more than 11 candidates have qualified since then, and several candidates have dropped out after finding that being pushed offstage badly hurt their ability to raise money or reach voters.

To get into the Dec. 19 debates, Democrats needed to get 200,000 unique donations and hit 4 percent in four national polls, or 6 percent in two polls of early states. Three Democrats reached the donor mark, but fell short on polling: Booker, Castro and Rep. Tulsi Gabbard (Hawaii). On Monday, Gabbard tweeted that she had “decided not to attend” the debate, having previously accused the DNC of bias by not counting several polls in which she hit the key numbers.

If the DNC relaxed its rules, and allowed candidates to qualify for debates through either polling or donations, it would also clear a path for former New York mayor Mike Bloomberg. While Steyer purchased Facebook ads urging Democrats to donate a few dollars and get him onstage, Bloomberg has refused to raise money, saying that self-funding his campaign makes him more accountable to voters.

“They’re using somebody else’s money and those other people expect something from them,” Bloomberg said of other Democrats in a Dec. 6 interview with CBS News. “Nobody gives you money if they don’t expect something, and I don’t want to be bought.”

Four other Democratic candidates have not qualified for recent debates: Sen. Michael F. Bennet (Colo.), former congressman John Delaney (Md.), author Marianne Williamson and former Massachusetts governor Deval Patrick. They were not approached to sign Saturday’s letter, which noted the “brief qualification window” for the next debate, on January.

“It’s vital to make modifications to qualification criteria now,” the nine candidates wrote. “Increasing access to the stage would make all of our candidates stronger and ensure that whoever ultimately emerges from this contest as our party’s nominee does so having proven themselves against the stiffest competition and having earned the trust of all Democratic voters along the way.”

DNC Chairman Tom Perez told The Post last month that the debate standards would change as Democrats began voting in early states. In her statement, Hinojosa said that criteria would reflect candidate strength “once voting starts in February,” though said nothing about the sole debate scheduled for next month.

This month’s debates are not without controversy, either. All seven of the candidates who qualified have threatened to boycott unless a labor dispute is resolved at its host, Loyola Marymount University.