I don’t think they threatened anything beyond enforcing existing law. Intimidating in its implied application, yes, but also not something that can be turned against them.
Is it the law in Missouri that people can be criminally prosecuted for libel/slander? Just asking as I don't know. If so that by itself would seem to be unconstitutional.
Also for public figures the bar is set very high for them to win in libel cases anyway so prosecution in such cases would be almost a waste of public resources. To me this is nothing more than an intimidation tactic directed more at regular citizens than political opponents.