Once the limit is reached, the Treasury Department would not be able to borrow as it does routinely to finance federal operations and roll over existing debt; ultimately it would be unable to pay off maturing debt, putting the United States government — the global standard-setter for creditworthiness — into default.

The repercussions in that event would be as much economic as political, rippling from the bond market into the lives of ordinary citizens through higher interest rates and financial uncertainty of the sort that the economy is only now overcoming, more than three years after the onset of the last recession.

Given the short time frame for action and the prospect of an intractable political clash, leaders in both government and business are already moving to avert a crisis that most likely would be “a recovery-ending event,” as Ben S. Bernanke, the Federal Reserve chairman, testified recently in the Senate. He described a sequence of events that “would cascade through the financial markets,” provoking another credit crisis like that in 2008 and causing interest rates to jump.

Mr. Geithner has been meeting privately with senior lawmakers of both parties to underscore the economic stakes. At the White House, Mr. Obama’s chief economic adviser, Gene Sperling, peeled away from the spending fight in recent weeks to turn nearly full time to developing the administration’s strategy for the debt-limit debate. Central to that, administration officials say, is whether Mr. Obama initiates bipartisan talks on a long-term debt-reduction plan that tackles taxes, military spending and fast-growing entitlement programs like Medicare and Medicaid.

Executives of the nation’s largest financial institutions in recent days met with Mr. Geithner, House Speaker John A. Boehner, Republican of Ohio, and other lawmakers, arguing for the importance of raising the debt ceiling. Jamie Dimon, the chief executive of JPMorgan Chase, told them that his bank had devised contingency plans to protect its global business in the event of a default.