Why does it say that he is becoming the third nominee of a major party in the nation’s history?

Question

He delivered it in a most unconventional setting, becoming the third nominee of a major party in the nation’s history to leave the site of his convention to give his acceptance speech at a stadium. In this case, it was Invesco Field, set against the Rockies and about a mile from the arena where he had been nominated the night before. His aides chose the stadium to signal a break from typical politics and to permit thousands of his supporters from across the country to hear him speak.

questions:1 why does it say that he is becoming the third nominee of a major party in the nation’s history . who were the first and the second.

2,the reason of 'leave the site of his convention'

thanks

Answer 1

<<Obama isn't the first candidate to accept the nomination at a huge outdoor venue. In 1960, John F. Kennedy described a ``New Frontier'' at Memorial Coliseum in Los Angeles before more than 50,000 people, and Franklin D. Roosevelt urged 100,000 listeners to seize their generation's "rendezvous with destiny'' at his 1936 speech at Franklin Field in Philadelphia.>>

Answer 2

Well if you are asking who else delivered a speech in a setting like that...John Kennedy did in Los Angeles.

Answer 3

I only know of Kennedy doing this.  I do not know who the other was.

They did it because the convention hall would not hold the crowd that wanted to see Obama accept the nomination.