S&P made it pretty clear that one of the reason for downgrade was the ineffectiveness of American political parties.
"More broadly, the downgrade reflects our view that the effectiveness,
stability, and predictability of American policymaking and political
institutions have weakened at a time of ongoing fiscal and economic
challenges to a degree more than we envisioned when we assigned a
negative outlook to the rating on April 18, 2011."
S&P | United States of America Long-Term Rating Lowered To 'AA+' Due To Political Risks, Rising Debt Burden; Outlook Negative | Americas
Yes, we do have the revenue to service our debt. However, servicing debt is not the only obligation the government has. Laws passed by congress has obligated the government to provide a multitude of services. When the congress doesn't act on either the budget or the debt ceiling, then the administration must decide which obligations are to be met and which aren't. Congress by lack of action chooses to let the administration make the decisions and the administration chooses what obligations are to be met.