Four Democratic presidential contenders have expressed concern about MSNBC co-hosting this week's debate unless Comcast, the parent company of the network, pledges to conduct an independent investigation of alleged sexual misconduct at NBC News.

The candidates — Sen. Cory Booker, D-N.J., Sen. Kamala Harris, D-Calif., Sen. Bernie Sanders, I-Vt., and Sen. Elizabeth Warren, D-Mass. — made their case in a letter addressed to Tom Perez, the chairman of the Democratic National Committee.

HuffPost first reported on the letter on Tuesday morning, noting that women's advocacy group UltraViolet had coordinated the effort. NBC News later obtained a copy, and the Booker, Harris, Sanders and Warren campaigns confirmed they had signed it.

UltraViolet had urged the DNC to pull out of the Wednesday night debate if Comcast decides not to open an independent probe, but none of the candidates threatened not to appear.

"We, the undersigned candidates, are very concerned about the message it would send to sexual assault survivors if our next debate is sponsored by MSNBC without clear commitments from Comcast … to conduct an independent investigation into the toxic culture that enabled abusers and silenced survivors," the letter says in part.

"It is critical that the Democratic National Committee make clear that they support survivors of sexual harassment and abuse by ensuring that Comcast and NBC News take steps to clean up the toxic culture that exists across their networks," the letter later says in part.

In May 2018, NBCUniversal — a unit of Comcast and the parent company of NBC News and MSNBC — released the results of a five-month internal investigation that the company said found no evidence the leaders of its news divisions were aware of allegations of sexual misconduct by former "Today" host Matt Lauer prior to his firing in November 2017. Lauer has vehemently denied allegations of nonconsensual sexual conduct.

The letter referred to misconduct allegations against unnamed "NBC executives and on-air talent." The company's internal investigation, led by NBCUniversal general counsel Kim Harris, in consultation with two outside law firms, found that NBC News did not have a broader problem with sexual harassment in its workplace.

"The investigation team does not believe that there is a widespread or systemic pattern of behavior that violates company policy or a culture of harassment in the News Division," the report said.

In a memo to staff members last month, NBC News Chairman Andrew Lack reiterated the findings of the company's investigation, adding that the suggestion that the company "tried to cover up any aspect of Lauer's conduct is absolutely false and offensive."

In a statement to The Hollywood Reporter last month, NBCUniversal spokeswoman Hilary Smith said no additional investigation would be launched, and that the company was "very confident in the report that was conducted."