There they stand in their nondescript suits, wires trailing from discreet earpieces. Unblinking eyes scan the crowd constantly. At any moment, they might have to throw themselves in front of a bullet, become a human shield. Of course, the real job of the Secret Service is to prevent just that scenario. Ex-Manson Family member Squeaky Fromme, dressed in head-to-toe red, was probably not that hard to spot and stop when she made her attempt on President Ford. But what about Arthur Bremer? Unable to get close enough to President Nixon, Bremer reset his sights on presidential hopeful George Wallace, whom he shot in nearby Laurel, Md. The model for Taxi Driver’s Travis Bickle, Bremer never would have called attention to himself by sporting a Mohawk. At the “Preventing Targeted Violence: Secret Service Approaches to Threat Assessment” lecture, the Secret Service will discuss how they set up the National Threat Assessment Center to prevent this kind of targeted violence. At 7 p.m. at Hirshhorn Museum and Sculpture Garden, Ring Auditorium, 7th & Independence Ave. SW. $15. (202) 833-9800. (Mark W. Sullivan)