Also notice how Obama won ~20% of Conservatives in 2008 and 2012. This is partially because a lot of black voters identify as conservative (primarily on cultural issues) but still vote Democrat.
Additionally, the number of self-identified moderates has dropped from 50% in 2000 as the number of liberals and conservatives rise. That's a combination of a greater focus on base turnout starting in 2004, as well as more people being honest about their true ideology.
The opposite is true for people calling themselves independents and Republicans; the GOP has become so embarrassing that many call themselves independent while still voting Republican, which is why Romney won the indy vote but still lost the election.
