Final answer:
Obama's primary victory was surprising due to the moderate success of the Democratic Party in the 1990s and Hillary Clinton's established political presence, but his message of change and adept use of social media and endorsements helped him overcome the odds.
Step-by-step explanation:
Barack Obama's defeat of Hillary Clinton in the Democratic primary of 2008 was so unlikely because through the 1990s, the Democratic Party was defined by more moderate, centrist policies, exemplified by the political success and style of Bill Clinton. During this era, the term "liberal" was often used as a pejorative, and self-identified moderates were prevalent. Hillary Clinton, who had been a prominent figure throughout the 1990s, held significant political clout and was initially seen as the frontrunner. Obama, a first-term senator from Illinois, did not have a long legislative record and was not as well known. Despite this, Obama's campaign employed innovative strategies and an appeal for change and hope that resonated with a wide array of voters. His success was further buoyed by endorsements from influential figures like Oprah Winfrey and the effective use of social media and technology to galvanize a base of youthful supporters.