Planning to commit a murder or a terrorist attack is a felony that will put an individual in prison for a long time.
Unless it is with other people and, more critically, at least one of the people involved goes beyond planning and takes some concrete step to advance the execution of the plan (at which point it becomes the separate crime of conspiracy), no, planning a crime, even of that seriousness, is not itself a crime.
https://en.m.wikipedia.org/wiki/Vikram_Buddhi
No other concrete step is needed.
Planning to kill someone is not a crime. Threatening to is, even if they aren't the President.