Nine Democratic presidential candidates, including the party's front-runners, are urging the Democratic National Committee to reopen televised debates to a broader pool of contenders to better reflect the historic diversity of the current field.

But party officials signaled Saturday that they are unlikely to budge and change the rules used in recent months to determine who makes the stage.

In the letter obtained by CBS News, the candidates say that the rules used in recent months to determine who will appear "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."

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"As a result," they add, "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."

Never before has such an emphasis been placed by Democrats on televised debates — and rules set by the party about who could appear compelled candidates to jump in, swelling the ranks of the field to as many as 25, including a record number of women and minorities.

The rules were set in the wake of the 2016 Democratic contest, following criticism by Senator Bernie Sanders of Vermont and his supporters, who said party leaders unfairly scheduled and limited the number of debates. That compelled DNC officials to establish a transparent and extensive debate schedule that would help winnow the field in the months leading up to caucuses and primaries.

Earlier this year, the party announced plans for 12 debates through the first months of the primary calendar. Four are scheduled for January and February in the first four states holding contests — Iowa, New Hampshire, Nevada and South Carolina. The final qualifications for those debates have yet to be announced.

The letter sent Saturday to DNC Chairman Tom Perez pushed him and top party leaders to revert back to the format used at the beginning of the debates, including rules that allowed a candidate to appear if they met certain polling or fundraising thresholds. In more recent months, rules requiring candidates to meet higher polling and fundraising levels than for previous debates have been designed to severely limit the number of participants.

But the nine candidates argue in their letter that "an unintended consequence" of the debate rules is that candidates who "helped make this year's primary field historically diverse" are being left off stage.

"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, who deserve to hear from and be able to choose among the best our party has to offer."

The letter is co-signed by Sanders, former vice president Joe Biden; Senator Elizabeth Warren of Massachusetts; South Bend, Indiana, Mayor Pete Buttigieg; Senator Amy Klobuchar of Minnesota; investor Tom Steyer; businessman Andrew Yang; former housing secretary Julian Castro and Senator Cory Booker of New Jersey.

Booker organized the letter and has been especially outspoken in expressing his concerns about the potential lack of diversity in the presidential contest as the new year begins, saying recently, "it is a problem that we now have an overall campaign for the 2020 presidency that has more billionaires in it than black people."

All nine candidates have appeared in debates so far this year, but Castro and Booker did not qualify to appear at the next debate this Thursday in Los Angeles. Former New York City Mayor Michael Bloomberg and former Massachusetts Governor Deval Patrick did not sign the letter because they have so far not qualified to appear in any debates.

Xóchitl Hinojosa, a DNC spokeswoman, said in a statement Saturday that party officials have "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."

Hinojosa added that the debate criteria is set to change again in February once voting starts, "which is more than appropriate."

If the party reverted back to the old formula suggested by candidates, it would likely mean two consecutive nights of debates, as was the case for the first round of Democratic debates. But the party does not have commitments from the broadcast and cable networks set to host forthcoming debates to air the events over two consecutive nights, according to party officials and network executives.

BuzzFeed News first reported the contents of the letter on Saturday.

The seven candidates who have qualified for the next debate are threatening to boycott the event at Loyola Marymount University in Los Angeles due to an ongoing labor dispute between the school's food service vendor and a labor union. All seven candidates set to appear have said they will not cross any picket line set up by the workers' union on campus and are calling for a resolution to the labor dispute.