Also Friday, McCain's team fired off an ad about Obama's relationship with Jim Johnson, the former Fannie Mae chairman who resigned as head of the Democrat's vice presidential vetting team after allegations that he had received preferential terms on a mortgage from Countrywide Financial.

CNN: Obama hammers McCain over Social Security

Early Friday, McCain's team returned fire with a commercial about Obama's relationship with Jim Johnson, the former Fannie Mae chairman who resigned as head of the Democrat's vice presidential vetting team after allegations he had received preferential terms on a mortgage from Countrywide Financial.

CNN: Economy takes center stage in bruising political ads