Not a lawyer either, but it's formally called The Brandenburg Test:

https://en.wikipedia.org/wiki/Brandenburg_v._Ohio

"The Court held that government cannot punish inflammatory speech unless that speech is 'directed to inciting or producing imminent lawless action and is likely to incite or produce such action.'"

One would want strong evidence for such a likelihood, of course. Evidence such as, for example, it having already done so three times before.

Steve Scalise's assassination was attempted by a radical anti-Trumper, Antifa has cracked open skills and sent people to the hospital. We have strong evidence that anti-Trump Facebook groups and subreddits cause real-world violence.