WASHINGTON — The United States moved perilously closer to an economy-rattling default and a partial government shutdown entered its third week as Senate Democratic and Republican leaders remained at odds over spending in their last-ditch negotiations to end the crises facing the nation.

Majority Leader Harry Reid, D-Nev., and Minority Leader Mitch McConnell, R-Ky., spoke by phone Sunday but failed to agree on a deal to raise the nation’s borrowing authority above the $16.7 trillion debt limit or reopen a government still shuttered on its 14th day. Congress is racing the clock with Treasury Secretary Jack Lew warning that the U.S. will quickly exhaust its ability to pay the bills on Thursday.

But there was no certainty that building anxiety among financial leaders around the world would provide the necessary jolt to Senate leaders, who represent the last, best chance for a resolution after talks between President Barack Obama and House Republican leaders collapsed.

Sen. Joe Manchin, D-W.Va., said today that investors are growing increasingly “skittish” about the possibility of default. The markets were closed for Columbus Day, but U.S. futures were down sharply. Earlier, trading in Asia was muted, with markets in Tokyo and Hong Kong closed for holidays.

The shutdown has furloughed 350,000 federal workers, impeded various government services, put continued operations of the federal courts in doubt and stopped the IRS from processing tax refunds. Some parks and monuments remain closed, drawing a protest at the National World War II Memorial on Sunday that included tea party-backed lawmakers who had unsuccessfully demanded defunding of President Barack Obama’s 3-year-old health care law in exchange for keeping the government open.

Economists see greater financial danger from an historical default. Christine Lagarde, the International Monetary Fund’s managing director, spoke fearfully about the disruption and uncertainty, warning on Sunday of a “risk of tipping, yet again, into recession” after the fitful recovery from 2008.

Reid and McConnell are at an impasse over the automatic, across-the-board spending cuts known as sequestration and whether to undo or change them as part of a budget deal.

Republicans want to keep the spending at the deficit-cutting level of the 2011 budget law while Democrats are pressing for a higher amount.

Politically, Republicans are reeling, bearing a substantial amount of the blame for the government shutdown and stalemate.

“We’re in a free-fall as Republicans, but Democrats are not far behind,” said Sen. Lindsey Graham, R-S.C., in warning Democrats about seizing on the GOP’s bruised brand as leverage to extract more concessions.

McConnell and Republicans want to continue current spending at $986.7 billion and leave untouched the new round of cuts on Jan. 15 that would reduce the amount to $967 billion. Democrats want to figure out a way to undo the reductions, plus a long-term extension of the debt limit increase and a short-term spending bill to reopen the government.