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As Oreo noted below, this "give a speech outdoors" thing has been done before - by John F. Kennedy in Los Angeles in 1960. Here are two articles from the NY Times. The first notes the venue was planned more than a year in advance, but it may not have helped (New York Times, PDF-subscription may be required):
Los Angeles, July 15, 1960 - Steps were started a year ago to insure tonight's convention wind-up at the Coliseum a basic audience of about 40,000-leaving 50,000 seats to be filled by spontaneity. [24,000 tickets were given out through local Democratic clubs - and there were 50,000 empty seats - Matt]The next time we see a blogger complain that the Wi-Fi isn't working somewhere, we'll remind them of how it used to be done!
The one-night location of the convention's press gallery in the midst of the Coliseum abyss necessitated the revival of the vanishing Morse dot-and-dash telegraph circuits. Technical problems made teleprinter installations difficult.
By almost sacred tradition, press telegraphers amplify the sound of their clickers by wedging Prince Albert tobacco cans into them. The cans can be bent so they give each clicker in a bunch of them a different sound. Occasionally, Edgeworth cans have been used, but their sound is definitely a minority report. So at the last minute, news organizations had the unusual convention chore of rounding up a flock of Prince Albert cans.
And the overall reviews were not kind: (New York Times, July 15, 1960, PDF-subscription may be required):
Los Angeles, July 15, 1960 - undismayed by the dim notices given their four-day dramatic flop inside the sports arena, the Democrats moved into the Coliseum today, to try their hand at fresh-air vaudeville.
For two and a half hours before the candidates arrived for their acceptance speeches, drum corps, bands, majorettes, movie performers, jazz musicians and comedians performed in a scorching sun on the turf where the Los Angeles Dodgers' outfield normally spends its evenings shagging flies.
The enormous stadium, which seats over 100,000, was little more than half filled when the candidates arrived.
The singing was just ending when a roar ascended from behind home-plate at the far end of the stadium, and in the might champions rode surrounded by their kin.
After they had ridden once around the ballpark to a standing ovation, they were led to their platform seats in the center-field bleachers.