"The majority of Republicans in Congress continue to resist any measure that would raise revenues, a position we believe Congress reinforced by passing the [debt-ceiling] act," S&P said.
Despite the explanation, Obama administration officials, lawmakers and financial industry leaders have hammered the company for what they say was an unnecessary move, based on a tenuous political calculus and slipshod economic analysis.
"They've handled themselves very poorly. And they've shown a stunning lack of knowledge about the basic U.S. fiscal budget math," Treasury Secretary Timothy Geithner told CNBC of the agency Sunday night. "And I think they drew exactly the wrong conclusion from this budget agreement."
Neither of the two other major U.S. credit rating agencies -- Moody's and Fitch -- has lowered its AAA rating for U.S. debt, although both have warned that future downgrades are possible.
David Beers, the head of the S&P's government debt-rating unit, told "Good Morning America" that he "absolutely" does not have second thoughts on downgrading the United States and warned that the nation's rating could get lowered below AA+ in the months ahead.