From an article in slate:
Embattled Sen. Rick Santorum, R-Pa., badly trailing in his re-election race, has decided to call in Bush to campaign for him in Pennsylvania. Problem is, Bush is one of the few politicians more unpopular in the state than Santorum. The senator, according to a recent Pennsylvania poll, has the approval of just 42 percent of Pennsylvanians and trails Democratic challenger Bob Casey by 14 points. The president, by contrast, enjoys the approval of 35 percent of Pennsylvanians.