Between John Kerry and George Bush, which one will look bad if a bunch of leftist idiots disrupt the convention?
Between John Kerry and George Bush, which one will look bad if a bunch of hoodlum leftist protestors start a riot outside Madison Square Garden?
Between John Kerry and George Bush, which one will suffer if it is found that some of the "protest" groups attempting to cause serious trouble had links to al-qaeda?
Chicago '68 did in Humphrey more than anything else. The public saw live on national television what the left was capable of starting and was disgusted. Southern conservative Democrats (many of whom were veterans) switched their support to Nixon and Wallace. That cost Humphrey the election.
Mayor Daley didn't HHH any good, either. I was living in Arlington Heights suburb at the time, and remember seeing busloads of disruptors coming in from all over the place.