Yellen warns lawmakers that the U.S. could breach its debt limit by Dec. 15.

WASHINGTON — Treasury Secretary Janet L. Yellen warned lawmakers on Tuesday that the United States would be unable to pay its bills soon after Dec. 15 and urged Congress to take swift action to lift the nation’s borrowing cap.

In a letter to Congress, Ms. Yellen said that “there are scenarios in which Treasury would be left with insufficient remaining resources to continue to finance the operations of the U.S. government beyond this date.”

She added: “To ensure the full faith and credit of the United States, it is critical that Congress raise or suspend the debt limit as soon as possible.”

Ms. Yellen sounded similar warnings just weeks ago. Last month, after approaching the first default in American history, Congress raised the statutory debt limit by $480 billion, an amount the Treasury Department estimated would allow the government to continue borrowing through early December.

But that was just a short-term fix and Republicans, who reluctantly joined Democrats in approving the temporary extension, have insisted that President Biden and his party vote on their own to approve a longer-term suspension or lifting of the debt limit.

Ms. Yellen has acknowledged that Democrats have the ability to address the debt limit without Republican support by using a budgetary procedure called reconciliation. However, she has said it would be preferable for the debt limit to be raised on a bipartisan basis — which has long been the tradition given the government must meet financial obligations that both Republicans and Democrats have incurred.

The Bipartisan Policy Center estimated late last month that the so-called X-date — the point at which the United States would be unable to fully meet all of its obligations like paying out Social Security and funding military paychecks — would most likely occur between mid-December and mid-February.

Last month, Ms. Yellen said at a congressional hearing that she supported eliminating the debt limit, which she believes is “destructive” and poses unnecessary risks to the economy.

In her letter to Congress, Ms. Yellen said that the recently passed infrastructure bill would require money to be transferred into the Highway Trust Fund within one month after the enactment of the legislation and that the transfer would be completed on Dec. 15. Shortly after that time, the funds will be invested in nonmarketable Treasury securities, which are subject to the debt limit.

Ms. Yellen noted that government cash flows were subject to “unavoidable variability” and that she would keep lawmakers apprised as the debt limit deadline changes.