The frustration in the air was palpable. Officials from the credit ratings agency Standard & Poor’s were meeting with Congressional leaders on a stifling late day in late July to discuss the thorniest issue in Washington: the effort to cut the nation’s deficit and raise the borrowing limit to avert a default. S.& P. and two financial industry groups listened to various proposals for debt reduction and warned the lawmakers of the impact a default would have on world markets, according to a Congressional staff member in attendance. The staff member said the agency was providing guidance on what target...