The Treasury Department could run out of money by Oct. 18, triggering a disastrous default on the nation’s debt that would damage the U.S. economy for years to come, Secretary Janet Yellen warned congressional leaders in a new letter on Tuesday morning.

Actions the agency is taking to skimp on cash to keep paying the government’s bills on time could be exhausted in just three weeks, she said. “At that point, we expect Treasury would be left with very limited resources that would be depleted quickly,” the secretary wrote. “It is uncertain whether we could continue to meet all the nation’s commitments after that date.”

The specific deadline comes as congressional leaders remain deadlocked in a standoff over raising or suspending the cap on how much the government can borrow. GOP leaders insist that Democrats must handle it on their own through the special budget process that they’re using to pass President Joe Biden’s multitrillion-dollar social spending bill — an untested route that could take roughly two weeks and pose a number of procedural issues.

Speaker Nancy Pelosi and Senate Majority Leader Chuck Schumer are privately discussing the options with Biden this week, although no decisions have been made.

Democrats, meanwhile, are daring Republicans to oppose bipartisan action that would stave off a debt default, which could roil financial markets, raise interest rates and whack the government’s credit rating.

Senate Republicans blocked a funding package on Monday night that would stave off a government shutdown at the end of the week and suspend the debt ceiling into December 2022. Schumer may force the GOP to vote a second time on that package, while House leaders consider passing a standalone stopgap spending bill and a separate debt limit measure in the coming days.

House Appropriations Chair Rosa DeLauro (D-Conn.) said Tuesday that she is “waiting to hear from the Senate on where they want to go“ with action to avoid a shutdown at the end of the week.

Those bicameral conversations will “continue until we come to a plan to see what the next steps are,” she said. “That’s where we are."

Yellen’s projection is more conservative than a recent estimate issued by the Bipartisan Policy Center, which said Congress could slam into a debt cliff between mid-October and Nov. 4.

The Oct. 18 deadline is Treasury’s “best estimate,” Yellen said, and is “subject to unavoidable variability“ while federal cash flow is in flux. The date could “unpredictably shift forward or backward,” she said.

“This uncertainty underscores the critical importance of not waiting to raise or suspend the debt limit,” she wrote. “The full faith and credit of the United States should not be put at risk.“

Jennifer Scholtes contributed to this report.

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