Standard & Poor’s downgraded the U.S.’s AAA credit rating for the first time, slamming the nation’s political process and criticizing lawmakers for failing to cut spending or raise revenue enough to reduce record budget deficits.

S&P lowered the U.S. one level to AA+ while keeping the outlook at “negative” as it becomes less confident Congress will end Bush-era tax cuts or tackle entitlements. The rating may be cut to AA within two years if spending reductions are lower than agreed to, interest rates rise or “new fiscal pressures” result in higher general government debt, the New York-based firm said yesterday.

“The downgrade reflects our opinion that the fiscal consolidation plan that Congress and the Administration recently agreed to falls short of what, in our view, would be necessary to stabilize the government’s medium-term debt dynamics,” S&P said in a statement late yesterday after markets closed.

Lawmakers agreed on Aug. 2 to raise the nation’s $14.3 trillion debt ceiling and put in place a plan to enforce $2.4 trillion in spending reductions over the next 10 years, less than the $4 trillion S&P had said it preferred. Even with the specter of a downgrade, demand for Treasuries surged as investors saw few alternatives amid concern global growth is slowing and Europe’s sovereign debt crisis is spreading.