Oh, he had a "list" of some sort that included POTUS...OMG CIRCLE THE WAGONS!!!
If the guy had a .338 rifle, a wind meter and a detailed itenerary for POTUS, I can see them getting a bit wound up about him. But this is just silly. Gay, even.
Well I would not go that far. Ronald Reagan was shot and nearly killed by John Hinckley Jr. with a .22 caliber pistol. Hardly a big bore magnum.
Lincoln was killed by a Derringer (.65 caliber I think) lethal only at point blank range.
McKinley was shot with a .32 caliber hand gun; again not an imposing weapon.
What makes a assassin credible is like you say is his planning capability and his likelihood of getting close enough.
If the guy is homeless I have to question his ability to plan. If you cant plan well enough to keep a roof over your head I doubt your ability to plan a credible assassination plot.
Most likely a 'Rat voting cheat sheet thrown down by the cops.