Here’s their answer:

You can limit political speech and associated conduct that are not work-related—provided you don’t infringe on protected Section 7 rights or applicable state laws. Section 7 of the National Labor Relations Act gives non-supervisory employees the right to talk about the terms and conditions of their employment and the right to unionize. While this law protects some political activities, it doesn’t give employees the right to discuss politics that aren’t work-related during work hours. 

That said, I recommend having a policy that focuses on job performance rather than political discussions specifically. If an employee spends too much time engaged in extra office chat, regardless of the topic, they’re probably not performing to your expectations. If nothing else, they’re distracting others. By prohibiting excessive chit-chat generally, you avoid creating the appearance of targeting political speech, and you reduce instances of other disruptive speech and behavior. 

You’re also certainly welcome to tell employees that all conversations should be held with indoor voices and that non-work-related topics should be reserved for break areas where they won’t be distracting those who need to focus.