If Congress fails to raise the credit limit, S&P says they will immediately drop the US bond rating from AAA to D, the lowest possible rating.
Moody’s says they would drop the rating to Aa.
Not raising the debt ceiling does not mean the US will default (unless Tim Geithner wants to). It might mean suspending payments to suppliers, not making payments into government pension funds, selling off government assets, and probably a dozen different accounting tricks. It would cause problems, but the highest priority thing the US Treasury would pay is interest on the debt.