This is an important point. Federal threat laws are intended to protect individuals from harassment via mail and telephone calls. A threat issued, a la Gordon Liddy, against unspecified individuals probably does not qualify. Similarly, conspiracy laws require an overt act because Americans tend to sit around envisioning all kinds of crimes they have no intention of committing; there are not nearly enough prisons in America to lock up every citizen who ever, in a private conversation, called for the assassination of a politician he or she did not like. In both cases, the laws are designed to define the border between protected and unprotected speech. You have a first amendment right to say some very repulsive things; but you may not be able to say them standing in front of the house of your enemy, when your audience is holding Molotov cocktails in their hands.