Standard & Poor's has cut the US credit rating for the first time in history, saying the country's politicians are increasingly unable to come to grips with its massive fiscal deficit and debt load.
S&P on Friday cut the US rating from its top-flight triple-A one notch to AA+, and added a negative outlook to it, saying there was a chance it could be downgraded again within two years if progress is not made cutting the huge government budget gap.
It said the "political brinkmanship" of recent months shows that governance in the country is becoming "less stable, less effective, and less predictable," raising the risks that it one day might not honour its debt.
It was the first time the US was downgraded since it received an AAA rating from Moody's in 1917; it has held the S&P rating since 1941.
The rating came after a strong pushback from the White House, which called S&P's analysis of the economy deeply flawed and politically-based.
A Treasury spokesperson alleged that there was a "two trillion dollar error" in the S&P analysis, arguing that the agency admittedly used the wrong baseline and erred on spending plans and debt projections.
But John Chambers, chairman of the S&P sovereign ratings committee, defended the decision.
"It's a matter of the medium and long-term budget position of the United States that needs to be brought under control," he said on CNN.
"This is a problem a long time in the making whether this administration and prior administration," he said.
He pointed to the White House, Democratic and Republican MPs battling for months until the country was on the precipice of default last Tuesday before they finally agreed to a deal to raise the nation's debt ceiling and slash the deficit.
The fight had sent jitters across the global economy.
A debt downgrade will be a symbolic embarrassment for President Barack Obama, his administration and the United States, and could raise the cost of US government borrowing.
There were also worries that the downgrade would wreak unpredictable havoc in global financial markets where the US dollar has long been the most important currency.
But some analysts believe the cut will not have much impact at all.
Indeed, despite a downgrade hanging overhead, the Treasury easily auctioned off tens of billions of dollars in new debt this week, and Treasury yields fell to the year's low.
S&P linked the downgrade directly to the stalemate in US politics.
Tuesday's fiscal consolidation plan "falls short of what, in our view, would be necessary to stabilise the government's medium-term debt dynamics," it said.
"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" back in April.
S&P said the negative outlook pointed to the possibility of lowering the rating to AA within two years if the political will to cut weakens or economic conditions worsen.
"Our opinion is that elected officials remain wary of tackling the structural issues required to effectively address the rising US public debt burden in a manner consistent with a 'AAA' rating."
S&P is considered the most influential of the three major rating agencies which also include Moody's and Fitch - both of which said this week that they continue to review the country's deficit reduction plan for possible downgrades.
S&P first warned Washington of a possible downgrade in April.
Then in July, during the protracted standoff over raising the government's debt ceiling between Obama and Republicans, S&P placed the United States on credit watch and warned of a possible cut within 90 days.
The agency indicated it would wait to see what the warring Republican and Democratic MPs came up with in their deficit reduction plan.
The plan finally agreed on Tuesday calls for $917 billion in cuts over 10 years, but also mandates an as-yet unnamed congressional panel to come up with another $1.5 trillion in cuts by the end of the years.
That fell short of what S&P has been saying would merit retaining the AAA rating: $4 trillion in deficit reduction over 10 years that includes both cuts and revenue increases, which Republicans have refused to accept.