Democratic presidential candidates, from left, Bernie Sanders, Martin O'Malley, Hillary Rodham Clinton and Lincoln Chafee stand on stage during the Iowa Democratic Party's Hall of Fame Dinner on July 17. (Charlie Neibergall/AP)

Mark your calendars: Democratic candidates for president will debate for the first time Oct. 13 in Nevada.

CNN and the Democratic National Committee, which will host the debate, announced it Thursday, just hours before Republicans will debate for the first time in Cleveland.

CNN did not say which candidates would participate. The network said the exact location would be released in the coming days.

DNC Chair Debbie Wasserman Schultz announced the dates and locations for the six Democratic debates, which will take place roughly monthly and begin in the four earliest voting states: Nevada, Iowa, New Hampshire and South Carolina. In response, some of the underdog candidates complained that there should be more debates.

Polls show Hillary Rodham Clinton is the clear Democratic front-runner. Sen. Bernie Sanders (Vt.) is viewed as her most formidable challenger. He has raised a substantial sum of money and is attracting big crowds as he travels across the country.

Also in the Democratic field are former Maryland governor Martin O'Malley, former Rhode Island governor Lincoln Chafee and former Virginia senator Jim Webb.

Meanwhile, there are signals that there is growing pressure on Vice President Biden to jump into the race.

O'Malley, who has been critical of the DNC for limiting the number of debates, took to Twitter Thursday morning to voice his frustration:

Sanders also expressed his displeasure.

"I am disappointed, but not surprised, by the debate schedule announced by the Democratic National Committee," he said in a statement. "At a time when many Americans are demoralized about politics and have given up on the political process, I think it's imperative that we have as many debates as possible -- certainly more than six. I look forward to working with the DNC to see if we can significantly expand the proposed debate schedule."

Here's the full Democratic debate schedule and sponsor list, per the DNC:

October 13: Nevada (CNN)
November 14: Des Moines, Iowa (CBS/KCCI/Des Moines Register)
December 19: Manchester, New Hampshire (ABC/WMUR)
January 17: Charleston, South Carolina (NBC/Congressional Black Caucus Institute)
February or March: Miami, Florida (Univision/Washington Post)
February or March: Wisconsin (PBS)