On the eve of Wednesday's Democratic debate on MSNBC, four presidential contenders called for an independent investigation into sister network NBC in the wake of allegations of sexual abuse.

The candidates, Sen. Elizabeth Warren, D-Mass., Sen. Bernie Sanders, I-Vt., Sen. Cory Booker, D-N.J., and Sen. Kamala Harris, D-Calif., demanded the inquiry in a letter to Democratic National Committee Chairman Tom Perez dated Monday.

"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, the parent company of NBC and MSNBC, to conduct an independent investigation into the toxic culture that enabled abusers and silenced survivors," the candidates wrote.

The candidates did not threaten to pull out of the debate if Comcast didn't make the commitment, though the letter could signal that they will raise the issue while on stage.

CNBC, MSNBC and NBC are all units of NBCUniversal, which is owned by Comcast. Representatives from Comcast and NBC News did not immediately respond to requests for comment.

MSNBC and The Washington Post are scheduled to host the next Democratic debate Wednesday in Atlanta.

The move comes in the wake of a book by investigative reporter Ronan Farrow alleging that NBC covered up abuse by former "TODAY" show co-host Matt Lauer and stymied reporting on film producer Harvey Weinstein.

NBC fired Lauer after an internal review found that he violated company policies. Lauer denied some of the allegations against him but said that "there is enough truth in these stories to make me feel embarrassed and ashamed."

Weinstein is awaiting trial in New York on rape and other sexual assault charges, to which he has pleaded not guilty. Farrow was awarded a Pulitzer Prize last year, alongside The New York Times, for work he published in The New Yorker detailing allegations of abuse against the movie mogul.

NBC News President Noah Oppenheim issued a memo to staff ahead of publication of Farrow's book, "Catch and Kill," which accused Farrow of distorting the facts.

"Ronan Farrow's book takes that undeniable fact and twists it into a lie — alleging we were a 'company with a lot of secrets.' We have no secrets and nothing to hide," Oppenheim wrote in the memo, which was obtained by multiple outlets.

NBCUniversal has faced scrutiny over its decision to investigate the allegations against Lauer internally. That investigation, overseen by the company's chief counsel, cleared management of wrongdoing. Other media organizations, including Fox News and CBS, have hired independent firms to conduct misconduct probes.

Farrow's book prompted some backlash even from within NBCUniversal.

Rachel Maddow, a popular MSNBC anchor, delivered a monologue in prime time in October questioning why the company did not contract out an independent investigation.

Chris Hayes, another MSNBC anchor, also challenged NBC from his television platform last month, suggesting the company took the "path of least resistance" that was the "enemy of the kind of work that we, as journalists, are supposed to do."

In their letter to Perez, which was earlier reported by HuffPost, the presidential contenders wrote that the party needed to set itself apart from President Donald Trump, who has been accused of sexual misconduct, including rape, by multiple women. Trump has denied the accusations.

"We, as a party, have to offer voters a clear and unquestionable difference come November when it comes to these important issues," they wrote. "We can not do that when we prop up and support companies that have systematically covered up numerous indicents [sic] of sexual violence with no accountability or changes of leadership."

The presidential contenders all either did not respond or declined to comment. The DNC did not immediately provide comment.

The full text of the letter is below.

Dear Chairman Perez: The latest allegations of sexual assault and harassment by former NBC employees perpetrated by NBC executives and on-air talent as well as a cover-up by NBC's management are deeply troubling. 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, the parent company of NBC and MSNBC, to conduct an independent investigation into the toxic culture that enabled abusers and silenced survivors. NBC's internal investigation last year ultimately concluded that because there were no formal complaints, NBC News executives were not at fault. Yet, there is clearly something wrong with a work environment reluctant to hold management accountable. That's what allowed the behavior of powerful abusers inside the company to go unchecked. The troubling reports about management's role to cover up abuse demonstrate that Comcast should have and needs to do more to shift the work culture and pursue significant structural changes in order to prevent future harassment and abuse at NBC and MSNBC. Until that happens, employees are at risk. Donald Trump has been credibly accused of sexual harrassment and sexual abuse by dozens of women. We, as a party, have to offer voters a clear and unquestionable difference come November when it comes to these important issues. We can not do that when we prop up and support companies that have systematically covered up numerous indicents of sexual violence with no accountability or changes of leadership. 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. Signed, Senator Cory Booker Senator Kamala Harris Senator Bernie Sanders Senator Elizabeth Warren

Disclosure: Comcast owns CNBC parent NBCUniversal.