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