Here is a paragraph from the S&P report on why they downgraded us:
Compared with previous projections, our revised base case scenario now
assumes that the 2001 and 2003 tax cuts, due to expire by the end of 2012,
remain in place. We have changed our assumption on this because the majority
of Republicans in Congress continue to resist any measure that would raise
revenues, a position we believe Congress reinforced by passing the act.
http://www.ft.com/cms/af2c4fac-bfc2-11e0-90d5-00144feabdc0.pdf (http://www.ft.com/cms/af2c4fac-bfc2-11e0-90d5-00144feabdc0.pdf)