The field narrowed to Obama and Senator Hillary Clinton after early contests, with the race remaining close throughout the primary process, but with Obama gaining a steady lead in pledged delegates due to better long-range planning, superior fundraising, dominant organizing in caucus states, and better exploitation of delegate allocation rules.[171]
On June 2, 2008, Obama had received enough votes to clinch his election. After an initial hesitation to concede, on June 7, Clinton ended her campaign and endorsed Obama.[172] On August 23, 2008, Obama announced his selection of Delaware Senator Joe Biden as his vice presidential running mate.[173] Obama selected Biden from a field speculated to include former Indiana Governor and Senator Evan Bayh and Virginia Governor Tim Kaine.[173] At the Democratic National Convention in Denver, Colorado, Hillary Clinton called for her supporters to endorse Obama, and she and Bill Clinton gave convention speeches in his support.[174][175] Obama delivered his acceptance speech at Invesco Field at Mile High stadium to a crowd of about eighty-four thousand; the speech was viewed by over three million people worldwide.