WASHINGTON (AP) ― The United States moved closer to a default that could seriously harm the economy 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 gripping the nation.

Democratic Majority Leader Harry Reid and Republican Minority Leader Mitch McConnell 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. Separately, they also could not agree on a plan to 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.

The reaction of world markets and the Dow Jones on Monday could 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.

The shutdown has furloughed 350,000 federal workers, impeded various government services, put continued operations of the federal courts in doubt and stopped the federal tax agency from processing tax refunds. Several parks and monuments remain closed, drawing a protest at the National World War II Memorial on Sunday that included conservative 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 ― five-term senators hardened by budget disputes and years of negotiations ― 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.

“I’m optimistic about the prospects for a positive conclusion to the issues before this country today,” Reid said as the Senate wrapped up a rare Sunday session.

McConnell insisted a solution was readily available as he embraced the proposal from a bipartisan group of 12 senators, led by Republican Sen. Susan Collins and Democratic Sen. Joe Manchin that would re-open the government and fund it at current levels for six months while raising the debt limit through Jan. 31.

It also would give agencies greater flexibility in dealing with the automatic budget cuts, delay the medical device tax for two years and establish income verification for individuals receiving subsidies to buy health insurance.

“It’s time for Democrat leaders to take ‘yes’ for an answer,” McConnell said in a statement.

But six Democrats in the group and a spokesman for Collins said late Sunday that while negotiations continued this weekend, there was no agreement.

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

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.

Unclear was whether any Senate deal would pass the Republican-controlled House by Thursday, though Senate Democrats were hoping momentum and an imminent default would pressure House lawmakers.

Graham appeared on ABC’s “This Week,” Corker was interviewed on “Fox News Sunday,” Schumer spoke on CBS’ “Face the Nation” and Lagarde was on NBC’s “Meet the Press.”