Why protecting politicians is hard

This opinion piece in the Washington Examiner sums up the difficulties that exist in keeping the political class safe. The attack on a group of Republican congress members was one in a string of assaults that have punctuated American history; think the Kennedys, Martin Luther King, Ronald Reagan, and even the Pope. Oftentimes, the attacker is hiding in plain site.
Politicians need to be accessible, and that often leaves them vulnerable. Moreover, protection extended to members of congress is much different from the security detail that follows the president.
Is more security called for? Of course, but that would take money, and in a time of fiscal belt tightening, seems unrealistic.