Gene Sperling , the director of the White House national economic council, called the difference, totaling over $2 trillion, “breathtaking” and said that “the amateurism it displayed” suggested “an institution starting with a conclusion and shaping any arguments to fit it.”

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Even as the ratings agency insisted on Saturday that its move shouldn’t have come as a shock, it reverberated around the world. Officials from China to Europe scrambled to assess the downgrade’s impact on the already troubled global economy, and political leaders in the United States sought to frame the issue in their favor.

Republican presidential candidates on Saturday seized on the downgrade as a new line of criticism against President Obama, suggesting that ultimate responsibility rests in the Oval Office.

“It happened on your watch, Mr. President,” Representative Michele Bachmann said, drawing applause at an afternoon rally in Iowa . “You were AWOL. You were missing in action.”

In a statement, the White House made no mention of the downgrade. “We must do better to make clear our nation’s will, capacity and commitment to work together to tackle our major fiscal and economic challenges,” the White House press secretary, Jay Carney , said.

The ratings agency’s action puts additional pressure on a still-to-be-named Congressional committee to find additional spending cuts, tax increases or both to bring down the inexorably rising national debt.

The debt-limit law agreement set spending caps in the fiscal year that begins Oct. 1 and calls for the bipartisan Congressional “supercommittee” to propose more deficit reduction — for up to $2.5 trillion in combined savings over a decade.

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Senate Majority Leader Harry Reid said the downgrade affirmed the need for the Democrats’ approach, balancing spending cuts with higher revenue from the wealthy and corporations.

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The decision, he said, “shows why leaders should appoint members who will approach the committee’s work with an open mind — instead of hardliners who have already ruled out the balanced approach that the markets and rating agencies like S.& P. are demanding.”

House Speaker John A. Boehner of Ohio , who runs the House with his anti-tax Republican majority, said that, “decades of reckless spending cannot be reversed immediately, especially when the Democrats who run Washington remain unwilling to make the tough choices required to put America on solid ground.”

While American politicians sparred, China, the largest foreign holder of United States debt, said on Saturday that Washington needed to “cure its addiction to debts” and “live within its means,” just hours after the S.& P. downgrade.

Europeans had girded for a possible downgrade, but the news was received with a degree of alarm in the corridors of power across the Continent.

Finance Minister François Baroin of France questioned the move Saturday, noting that neither Moody’s nor Fitch , the two other major ratings agencies, had reached a similar conclusion.

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The downgrade could lead investors to demand higher interest rates from the federal government and other borrowers, raising costs for local governments, businesses and home buyers.

The wrangling over S.& P.’s downgrade to AA+ from AAA stretched on for days. But interviews with both officials from the administration and S.& P. reveal sharply differing perceptions on whether a downgrade was imminent. The rating agency argued that their intentions had been plain for months if the government didn’t take strong action to curb the debt; administration officials claimed they were blindsided.

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The drama, which would culminate late Friday and into the weekend, actually began to gather speed Wednesday, when S.& P. executives came to the Treasury Department to meet with a group of administration officials led by Mary J. Miller, the assistant secretary for financial markets.

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At the meeting, the S.& P. executives walked the Treasury Department team through its analysis. Government debt was growing rapidly, they said, and the just-completed deal wasn’t going to do enough to slow it down, endangering the AAA rating.

As early as April, S.& P. had changed its credit outlook on the United States to negative. By July, S.& P. warned that if the government did not agree to a deficit reduction package of about $4 trillion, there was a one-in-two chance a downgrade.

Still Treasury officials claim they were taken by surprise on Wednesday. Just the day before, Ms. Miller and her team met at the Hay-Adams Hotel with a group of senior Wall Street executives who advise the Treasury on its borrowing. None of the members believed that the government’s credit rating would be lowered in the near-term.

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On Thursday, the ratings agency informed the Treasury that its seven-person panel would meet Friday morning to assess the creditworthiness of the United States government.

Even then, one administration official said, “We didn’t think they would actually do it.”

At 8 a.m. Friday, S.& P. convened a global conference call of its sovereign rating committee including Mr. Beers, Mr. Chambers and others. By 10 a.m., they’d reached a majority decision — the United States no longer was entitled to its top rating. Mr. Beers would not say whether the verdict was unanimous.

Rumors of a downgrade were already swirling in the markets — a prime reason the Dow dove more than 200 points at lunchtime, and at 1:15 p.m., the three men called the Treasury to inform them of the decision. “They were not pleased with the news,” Mr. Beers said.

Half an hour later, Treasury Secretary Timothy F. Geithner called William M. Daley , the White House chief of staff, as well as Mr. Sperling, according to administration officials. They delivered the news to President Obama in the Oval Office, before he took off to Camp David for the weekend.

Inside Treasury, meanwhile, John Bellows, an acting assistant secretary, flagged a concern over S.& P.’s methodology. In its analysis, S.& P. had projected the nation’s debt as a share of gross domestic product to reach 93 percent by 2021. That was around 8 percentage points higher than the figure administration officials believed the rating agency should have used — what they now call a $2.1 trillion error.

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In a Treasury blog entry, Mr. Bellows wrote that the difference raised “fundamental questions about the credibility and integrity of S.& P.’s ratings action.”

Around 5:30 p.m., S.& P. officials called the group of Treasury officials. “You were right,” Mr. Chambers told them, but said he was prepared to proceed because the revisions didn’t meaningfully affect S.& P.’s conclusion.

In one final effort to prevent what was once unthinkable from becoming inevitable, the Treasury officials again pressed S.& P. to reconsider. At 8 p.m., the ratings agency sent them the final press release on the downgrade. By 8:20 p.m., the news was out.

“For those who follow the fiscal situation of the United States, this shouldn’t be news to anyone,” Mr. Chambers said.