A group led by prominent Democratic legal and national security experts is spending six figures on a new television ad backing special counsel Robert Mueller’s investigation into President Donald Trump’s ties to Russia, following the forced departure of Attorney General Jeff Sessions.

Sessions resigned Wednesday at the request of the president, who named Matt Whitaker as acting attorney general now overseeing the Russia investigation instead of Deputy Attorney General Rod Rosenstein. Democratic lawmakers and others fear that Whitaker, a Trump loyalist, might soon move to dismiss Mueller and end the investigation.

The 30-second ad from Protect the Investigation will start airing on cable networks Friday and is part of a six-figure ad purchase.

“Democracy can’t exist in the dark,” a male narrator says in the ad. “But that’s where this president wants to leave us. Tell Congress to protect the Mueller investigation.”

Protect the Investigation is led by an advisory board including a number of prominent Democratic legal and national security experts, including officials at the Center for American Progress, the Leadership Conference on Human and Civil Rights and the Truman National Security Project. Former CIA analyst Ned Price, a top aide to President Barack Obama who is now the director of policy and communication at National Security Action, is among the advisers.

“The investigation into Russian meddling has already resulted in almost 200 criminal charges and eight guilty pleas,” Price said. “That’s why an attack on it by the president is an attack on our democratic values and the rule of law. The American people deserve to know the truth, as embodied in the results of the special counsel’s investigation — and they will hold lawmakers accountable.”

The ad’s launch comes the morning after thousands protested in Chicago, New York, Washington, D.C., and elsewhere asking Congress to protect Mueller.