At one point, Special Agent Butterfield, the head of Bartlet's Detail, informs Charlie that, based on the third suspect's statements, he (Charlie) was the target. Butterfield also correctly notes that the types of weapons used in the shooting were wrong. The reason is that the distance from the shooters to the target were way out of a pistol's range; something with a longer barrel, like a carbine or an assault rifle, would have been more appropriate. Even a 0.22-gauge hunting rifle would have been a better choice, as it would have been accurate at that distance.

The question, though, is this: if they were shooting at Charlie, why pick that moment? Charlie has no protection at other times, like when he goes to and from work, or out on errands, or hangs out on the weekend. (Yeah, when he's with Zoey, he falls under the watch of her Detail. But not other times.) These guys want to pop Charlie and yet they choose to go after him at a point in time when security is heaviest. The Secret Service is most alert (and nervous) when the President is outdoors, like he was when the shooting occurred. He and his party were absolutely blanketed with agents, not to mention that there was a large number of local cops backstopping the Service. Yet, they set up the hit at this time, using the wrong weapons nonetheless. It makes no sense.

There's a flashback scene to where Bartlet comes to visit Josh at the airport as he waits to fly home after his father dies. Four security men, looking just like Secret Service, walk up to Josh and box the area in. Then Bartlet shows up. Question is, who are these security guys? Presidential candidates aren't entitled to Secret Service protection until they're nominees of one of the two major parties, and at this point, that hasn't happened. Bartlet's protection should be handled by the State of New Hampshire, as he's still the Governor (right? or not?); his detail should be made up of troopers from the New Hampshire State Police. But, since police officers do not have jurisdiction outside of their state, it's customary for the "receiving" state to provide police protection for visiting state officials from another state.

I thought that was the evening when he clinched the nomination?

In any case, the Secret Service are obligated to protect the Democrat and Republican nominee but they can alsobe assigned earlier. I remember Obama received S.S. protection before being nominated.