Lynn Vavreck finds that Obama's popularity has fallen but that he's not alone:

Obama still wins in [2012 presidential] contests between all three of the Republicans we asked about - Romney, Palin, and Gingrich.  Against Romney, Obama’s loss in vote share from 2009 is seven points, from 48 to 41 percent – so yes, Obama has lost votes this last year.   But so, too, has everyone else.  Romney’s share of the vote is down 10-points, from 42 to 32; and Gingrich’s dropped nine points, from 39 to 30.  Palin loses the fewest votes, but she was starting from an already low baseline – 35.2 in 2009 to 32.4 in 2010. 

Voters like all the candidates less than they did a year ago, not just Barack Obama. 

More context on his historically solid approval numbers and the previous parallels (Reagan and Clinton) to what seems about to happen next week here.