Whatever it may be; at the time of this writing, it's the fence-jumper with the knife, but by the time you read this there may be another one.
Anyway: this brings to mind one of my less accurate predictions about Obama, which was this: he wouldn't finish his first term. Either he'd get bored around the two-year mark and wander off in search of higher office, like Queen of the Space Unicorns, or the Chicago Mob would decide he'd be more valuable as a martyr than as a live distraction, and have him bumped off ahead of an election.
Well, he's still alive (unless he's been replaced by a Disney-made audioanimatronic contrivance, which would miss the point entirely), but we're coming up on a midterm election, and then there's 2016, so there are still two opportunities for martyrdom.
And an ineffectual Secret Service might just be part of the grand plan. Makes it easier for some random loser to do the deed. (Of course, said loser mustn't be taken alive. Can't have him shouting about single-payer, or snackbars, or whatever, when the propaganda machine is trying to label him as a Tea Partier.)
Come to think of it, 2016 would be a better time; we'll be clearly at war then, and having some fanatic (of whatever stripe; we can count on his fanaticism being misidentified anyway) kill the wartime President would surely give Her Inevitableness a big boost. Also, we'd only have 3-4 months of Plugs.
Or, maybe the decline of the Secret Service is just another symptom of general decay, and not part of any specific plot. Gramsci, anyone?