Extra security might have made a difference and perhaps not. Kennedy had the image of a populist and insisted on pressing flesh.
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You may be right...I don't know. I was thinking possibly another pair of eyes may have spotted Sirhan Sirhan sooner, or something. But if I were the one who decided to decline the extra security, I would have carried around some rather heavy guilt after Kennedy was shot.
As was pointed out, the Kennedys were akin to the Republican conservatives of today from a political standpoint. Now as to personal morals, well, that's another story.