S&P said it was now "pessimistic about the capacity of Congress and the administration to be able to leverage their agreement this week into a broader fiscal consolidation plan that stabilizes the government's debt dynamics anytime soon."
One analyst suggested the downgrade might move Congress to take concrete steps to fix the nation's budget problems.
"It's a downgrade and it's bad, but if it spurs more conversation about bringing down spending and maybe more intelligent tax policy, it could be a good thing in the long run," said Frank Barbera, a portfolio manager of the Sierra Core Retirement Fund.
The Federal Reserve and other U.S. regulators said in a joint statement that S&P's action should not have any impact on how banks and other financial institutions assess the riskiness of Treasurys or other securities guaranteed by the U.S. government. The statement was issued to make sure banks did not feel that the downgrade would affect the amount of capital that regulators require the banks to hold against possible losses.
Before leaving for a weekend at Camp David, President Barack Obama met with Treasury Secretary Timothy Geithner in the Oval Office late Friday afternoon.
The downgrade is likely to have little to no impact on how the United States finances its borrowing, through the sale of Treasury bonds, bills and notes. This week's buying proves that.
"Investors have voted and are saying the U.S. is going to pay them," said Mark Zandi, chief economist of Moody's Analytics. "U.S. Treasurys are still the gold standard." He noted that neither his parent organization, Moody's, nor Fitch, the other of the three major rating agencies, have downgraded U.S. debt.
The ratings agencies were sharply criticized after the financial crisis in 2008 for not warning investors about the risks of subprime mortgages. Those mortgages were packaged as securities and sold to investors who lost billions of dollars when the loans went bad.
Japan had its ratings cut a decade ago to AA, and it didn't have much lasting impact. The credit ratings of both Canada and Australia have also been downgraded over time, without much lasting damage.
"I don't think it's going to amount to a lot," said Peter Morici, a University of Maryland business economist.
Still, he said, "The United States deserves to have this happen," because of its clumsy handling of fiscal policy.
In reacting to the downgrade, Democrats and Republicans continued to blame each other and pledged to hold firm to their principles.
Republican presidential candidates criticized the White House. Rep. Michele Bachmann, R-Minn., called on Obama to fire Treasury Secretary Timothy Geithner and submit a plan to balance the budget and not just reduce future deficits. Republican candidate Mitt Romney, former governor of Massachusetts, said the credit downgrade was the "latest casualty" in Obama's failed economic leadership.
House Democratic Leader Nancy Pelosi said the American people will be closely watching the work of the 12-member joint committee that has been created to produce more than $1 trillion in additional savings over the next decade.
"The work of this committee will affect all Americans, and its deliberations should be open to the press, to the public and webcast," she said.
Senate Democratic Leader Harry Reid said the downgrade underscored the need for a "balanced approach to deficit reduction that combines spending cuts with revenue-raising measures" such as doing away with tax breaks for the wealthy and oil companies.
AP reporters Tom Raum, David Espo and Julie Pace in Washington and Business Writers Chip Cutter and Pallavi Gogoi in New York contributed to this report.
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