Gallup notes that despite a stated desire to bring the country together, Barack Obama represents a continuation of a trend in which views of incumbent presidents are increasingly polarized:
As a candidate and as president, Obama — like his immediate predecessor, Bush — sought to bring Americans together after periods of heightened political polarization in the United States. But despite their best intentions and efforts, both men’s approval ratings have been characterized by extreme partisanship — with high and seemingly unwavering approval from their own party’s supporters and very little from the opposition party.
The way Americans view presidents has clearly changed in recent decades, perhaps owing to the growth in variety, sources, and even politicization of news on cable television and the Internet, and the continuing popularity of politically oriented talk radio. The outcome is that Americans evaluate their presidents and other political leaders through increasingly thick partisan lenses.
That media-centric causal explanation doesn’t strike me as even remotely plausible. The total audience for cable news is extremely small in the scheme of things. And you could say the same for political blogs.
A much more plausible theory is that it used to be the case that white southerners nearly universally identified as Democrats even while holding diverse views on many specific issues. This created a reservoir of Democrats who might disapprove of Democratic presidents or approve of Republican presidents. In a related manner, at the time many northeastern Republicans had fairly progressive views on a variety of issues and most northern Republicans were at least somewhat progressive on civil rights. But that old party system dissolved in the seventies and eighties, leaving Bill Clinton and George W Bush and now Barack Obama to govern in a world where the parties are ideologically coherent and partisans are therefore very unlikely to approve of an opposite-party president.