Jason Pargin made the point in his book 'I'm starting to worry about this black box of doom' (I'll probably be quoting it for a while) that there's little actual point in assassinating a US president because they'll be immediately suceeded by their VP who will have the exact same ideology. Pargin argues that it makes a lot more sense to kill supreme court members, since they are 'probably the closest thing the US has to royalty' --their word is near-impossible to overturn, and once appointed, they rule for life: If you don't like a SCOTUS judge, you can't just decide to replace them with a new one. On the other hand, off one, and you can appoint a new judge who might be more in line with your ideology.
(Not saying I (edit: or Pargin) encourage anyone to kill SCOTUS members, of course.)