The fact that they include former first lady Barbara Bush (Barbara Bush !!!! ???) makes it a federal crime, which the Secret Service is empowered to not only investigate but to actively intervene to prevent them from carrying out their threats, without further court action.
They can, for example, enter and search the premises of these people (offices or homes) without a specific court issued warrant based solely on this threat. They can also detain these individuals indefinitely, so long as they consider them to be a threat to the proetected person.
Not to worry. A friend of mine once told me that any organization with at least 2 people, one is an FBI agent, lol.