I don't think a good portion of people were voting FOR Obama in the 2008 election so much as either voting against Bush or voting for a historical election result (it was the first time the black population voted at a higher rate than the population as a whole). A lot of people didn't really know what to expect from him once he took office; sure, it was all there in his campaign, but I really don't think people were voting issues in 2008. When the PPACA was pushed through, it was a wholly unpopular piece of legislation with certain groups, as it was bound to be, but then that dislike spread to the population as a whole by the time the 2012 elections were ramping up.
Add to that, President Obama would needlessly insert himself into divisive racial issues, specifically with regards to the cop and the Professor Gates issue in Cambridge and later the Trayvon Martin case, and clearly taking sides before all the facts were made available. Why would he ever insert himself into local police matters? Anytime he was asked about them, he should have simply refrained. Publicly taking sides, IMO, ignited those who were already against him, and likely cooled some peoples' moderately warm outlook on him as president.