drew and heath are right.
in 2004 during the election campaign, bush came to my area to give a speech, and i along with some of my reporters and photographers covered it for my university paper.
just to be on the press pass list, i had my background vetted by the fbi and the secret service and needed a letter from the university naming me as an editor of the paper.
when we got within a few miles of the location, they had roadblockes set up with armed police and swat members, and the entire parking lot around the event center was blocked off, everyone had to park in a remote lot and take a bus or walk to the actual building. the entire way between the lot and the event was lined with police
once there, i had to drop off my bag, my photog's bag, and any other accessories with the secret service where they were searched and sniffed by bomb dogs. then we passed through multiple security checkpoints with guards, handheld and freestanding metal detectors, and we needed multiple forms of id along with press credentials just to get through each checkpoint.
inside, there were secret service and tons of police stationed in full sight, and it is a safe bet to assume that there were several plainclothes agents and policemen in the crowd as well, not to mention the 6 or so agents that surrounded bush as he walked in and out of the room. when he was speaking, they were never more than an arms reach away, scanning the room.
im not saying that it is impossible for our pres. to be killed while in office, it would just take the work of a trained professional using a well executed plan drawn up using inside knowledge of the security around the president, not some meth heads with a deer rifle.