The people who hate Bush are generally the stupid democrats who hate guns. Thus, the body of people most likely to assassinate him can't assassinate him because they are generally not gun owners. If a person does own a gun they are more likely to be favorable toward gun rights, and thus more likely to be favorable of Bush.
So democrats, while they hate Bush, are less likely to own guns due to their pacifism and support of gun control.
Republicans, who tend to own guns and are thus more likely to be in a position to assassinate him, for obvious reasons don't dislike him quite as much as the democrats, and are thus significantly less likely to assassinate him than the dems.
In short, the democrats are pussies, and the republicans are either indifferent to or supportive of the pres.