Most people either know of or know the candidate before he even runs. No one with the exception of the people in his district in Chicago even knew who he was and I'm reaching on this considering most in that district were oppressed.....and stayed that way after he destroyed it.
he was the keynote speaker in 2004. Enough people in the DNC thought he had the potential to win.
He's a kenyan communist, but back in 2004, he was the most shiny dem in their puddle of garbage water. It was polarizing hated hilary, or nobody. He had been in the Senate. So he was pretty well known in DC.
In this day of media, it wasn't hard, from 2004 (when the DNC probably decided he was THE anti-hilary choice) to 2007, to make him very popular. People learn fast. I mean, Palin was MUCH less known, being governor of a very low-population state for a year, and VOILA, overnight, she was a national star.