This is the second in a three-part series on the world of campaign ads. In this installment, Fox News examines the evolution of the industry over the decades. 

It only aired once -- and yet it remains arguably the most memorable and influential political ad of all time: "Daisy," the 60-second spot in which President Lyndon Johnson conjured the specter of nuclear annihilation to frighten voters from backing his 1964 presidential campaign opponent, Republican Sen. Barry Goldwater of Arizona. 

The black-and-white ad featured an adorable two-year-old girl standing in a meadow, counting off petals as she picked them from a daisy flower, endearingly getting her numbers wrong.  When the last petal has been picked, the image abruptly freezes and zooms into the black hole of the little girl's eye, while a stern male voice commences a countdown to zero. Then -- a horrifying explosion. With the entire screen consumed by the mushroom-cloud imagery of a nuclear detonation, we hear LBJ's voice, never mentioning Goldwater by name, intoning: "These are the stakes..."