“Fox News is in the midst of a critical ad sales period where they need to sell 60-70% of all their ad inventory for next year. But, due in large part to concerted efforts of activists and organizations, Fox News is having an ad sales crisis and is not able to fully persuade the advertising community that it is safe to come back in the water," he said.

“Doing this town hall in the middle of this moment will be used by Fox News to try and assuage advertisers concerns in a way that doing the town hall in a few weeks from now would not have done,” Carusone added.

Marianne Gambelli, the president of ad sales at Fox, said in a statement to The Hill that Fox News "had a very successful upfront presentation where our advertisers walked away feeling extremely positive about our story and the value of our audience."

"We expect no change in our business going forward," Gambelli added.

Sanders is scheduled to appear in a town hall on April 15 that will focus on the economy and jobs. It will be co-moderated by Fox News anchors Bret Baier and Martha MacCallum.

Sanders told HuffPost he believes Fox pushes an agenda, but stressed the importance of reaching the network's viewers.

-- Updated at 3:21 p.m.