Such an event would be unprecedented, and many financial analysts fear a possible violent market reaction with global ramifications.

Privately, economic officials have worried that the numerous near-brushes with a debt ceiling or other budgetary crisis in the last few years have inured the markets, with investors expecting Washington to wait until the last minute before acting.

If that expectation were to prove wrong, though, the effects are unknowable – and might be severe. “Any delay in raising the debt ceiling would have dire economic consequences,” said Mark Zandi of Moody’s Analytics, testifying on Capitol Hill this week. “Consumer, business and investor confidence would be hit hard, putting stock, bond and other financial markets into turmoil.”

In the event of a default, the United States’ borrowing costs would probably rise and continue at somewhat higher levels even after the Treasury Department returned to new issuance on the bond market, causing a direct hit to taxpayers. But financial analysts are more immediately worried about the potential for wide market gyrations as investors reassessed their pricing of trillions of dollars of debt products tied to Treasury rates and sought safety in new markets or instruments.

The costs from a debt-ceiling default would almost certainly dwarf the costs associated with a government shutdown, which most experts say would be relatively small if it did not continue for an extended period of time. The Bipartisan Policy Center, a Washington research group, estimated that market concern over the potential of a default in 2011 cost nearly $19 billion over 10 years, and that occurred even though the government avoided a default at the last minute.

The House recently passed legislation that would order Treasury to prioritize payments to bondholders — an action meant to soothe the markets in the event of a debt-ceiling crisis. But the Obama administration has rejected that idea and refused to negotiate over the debt limit more broadly.

“There is no way of knowing the damage any prioritization plan would have on our economy and financial markets,” Mr. Lew wrote. “It would represent an irresponsible retreat from a core American value: We are a nation that honors all of its commitments.”