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Independent experts ranging from Federal Reserve Chairman Ben S. Bernanke to former Fed Chairman Alan Greenspan and the International Monetary Fund also have raised dire scenarios about a debt crisis that could be only a month away, given the current impasse.

According to Treasury’s calculations, Congress must have passed an increase in borrowing authority by Aug. 2 to avoid the possibility of default. But because of the many conditions that Republican legislators are attaching to increasing the debt — generally requiring equal or greater cuts in spending and no tax increases — legislative aides said an agreement must be forged by July 22, to allow time for votes in the House and Senate and beat that deadline.

“A debt default in the U.S. government debt market would have very serious, far-reaching, dramatic repercussions, and that’s why we’re confident that it will be avoided,” said John Lipsky, the IMF’s acting managing director, on Wednesday.

But he warned against even a lesser misstep, saying that “a loss of fiscal credibility could cause an increase in interest rates or even potentially a sovereign downgrade, and that would have significant repercussions here and elsewhere.”

According to the rating agencies and other Wall Street analysts, the stakes go far beyond just how easily and cheaply U.S. citizens, Congress and Treasury can borrow in the future.

They say a large swath of the U.S. economy and financial markets also would be shaken even by a relatively small downgrade of the U.S. government because it would trigger a cascade of credit downgrades on other vital institutions.

Among the pillars of the economy whose credit ratings would be threatened are numerous state and local governments, major banks, mortgage giants Fannie Mae and Freddie Mac, the Federal Home Loan Banks and Federal Farm Credit Banks. A downgrade of Fannie Mae or other mainstays in the housing and credit sectors that are still struggling from the recession could be particularly devastating, analysts say.

Moody’s, which said it may move on the U.S. rating by mid-July if Congress continues to dawdle over the debt limit, is reviewing institutions throughout the U.S. economy to determine how dependent they are on funding from the federal government and thus how vulnerable they would be to a default on U.S. obligations.

“Some Aaa ratings of state and local governments could be vulnerable to credit pressure where sovereign credit linkages are potentially strong,” the Wall Street agency said in a statement Wednesday. But it added that the handful of U.S. corporations, such as Exxon Mobil and Johnson & Johnson, that have earned AAA ratings on their own, not through linkages to the government, likely would be unaffected.

Some Republican legislators have argued that a default can be avoided once the Treasury hits its debt ceiling next month if the government gives top priority to paying interest on the debt and delays payment of other government obligations until the impasse is resolved.

But a study by the Bipartisan Policy Center last week said that strategy would backfire and still endanger the government’s credit rating.

Treasury, which cuts about 80 million checks to pay the government’s bills in a typical month, could exhaust all of the revenues it is due to receive in August by paying only six major items: interest on the existing debt, Medicare, Medicaid, Social Security, unemployment insurance and defense contracts, said the study’s author, Jay Powell, a former undersecretary of the Treasury under President George H.W. Bush.

That would leave no money to fund entire U.S. departments, such as Justice, Labor and Commerce, he said, and there would be no funds to pay for veterans’ benefits, IRS refunds, military active-duty pay, federal salaries and benefits, special-education programs, Pell Grants for college students, or food and rent payments for the poor.

“The choices would not be pretty,” Mr. Powell said. Moreover, Treasury’s ability to roll over $500 billion of securities that mature during August would be in peril, he said, likely triggering a downgrade even if Treasury strives to make all interest payments.