And I wonder if the Regressives would consider "trying to convince as many Americans as possible that their President is an insane, out of control, white supremacist" might be considered "harmful"?

.
We have a CONSTITUTION and a GOVERNMENT to determine what is harmful. Harley doesn't want the government to have any such authority. Nor do you apparently.
With respect, I'm not sure "harmful" is the correct analysis. We could differ on whether NFL players kneeling is harmful, or whether it is harmful to knowingly post false facts on social media. But the gummit should not be able to punish that. Facebook can limit what people post though.
What TN and his ilk (I've always wanted to use that LOL) seem to miss is that when one exercises a constitutional right, one can affect another's exercise of his own right. Free speech analysis in a school setting is different than the analysis for adults who are just in the workplace and public settings. There's the too often used phrase "you don't have the right to cry fire in a crowded theatre." But oddly you do have a right to parade with Nazi flags and salutes in front of a synagogue. There seems to be a distinction there, but I find it hard to grasp. I get it that I can burn an American flag, and in theory a cop is supposed to not let a bunch of people kick my ass for being a jerk who's burning a flag. But there I'm not threatening anyone. A bunch of Nazi's with clubs and guns is threatening.
But in theory we have a right of "peaceable assembly." And we have a right to comment on society. However, like the right to own guns ... no right is absolute, and no one has an absolute right to affect others' enjoyment of their rights.
Speech can be regulated, and one doesn't have a right to a job unless they have a contract, and even then a contract can condition your right to employment based on what you do