Here’s some data-free speculation based on anecdotal information. If you look at the actual voting base of the Democratic Party — not left-wing intellectuals, but minorities and poor people — one of the biggest factors driving lack of enthusiasm for Barack Obama is Barack Obama’s mild unpopularity. People really liked the fact that other people liked Barack Obama. It made them feel better about America and about themselves. It made them feel like they were on the winning team politically, and the right side of history more broadly.
People who previously had been jazzed about voting for Hillary Clinton got enthusiastic about Obama once she dropped out and he was beating McCain — if he’d been losing, they would have been whining about how Hillary would do better. Mass political behavior is mostly expressive, not instrumental. What the fans want is for the team to win. Highly ideological political obsessives have more complicated concerns than that, but your “base” is your fans.