Contrary to what many people think, the bar is actually really very high for what constitutes a threat to an elected official. There was a case out in California a few years back where someone posted that that they would like to kill the President. They were charged, and convicted, but then the Appeals Court threw out the conviction saying a mere threat wasn't enough and the Federal Government had to also prove they had the intent and the ability to carry out the threat. So the law is currently, you can threaten the President, but as long as you just say you had no intent to carry it out(and the government can't prove otherwise), There is not a whole lot they can do to you...
We won’t know if he is a legitimate threat unless he and his home and belongings are investigated.