The United States faces a default sometime between Dec. 21 and Jan. 28 if Congress does not act to raise or suspend the debt ceiling, a Washington think tank warned on Friday.

The projection from the think tank, the Bipartisan Policy Center, was a narrower window than it provided last month, and the nonpartisan group suggested that the actual deadline, or X-date, could be toward the earlier end of that range.

Democrats and Republicans appear to have tempered their tone around raising the debt limit this time around. While lawmakers have not settled on a path to lifting the borrowing cap, they are exploring a series of ways to raise it, including some that could ultimately hand more power to the White House to avoid the kind of standoffs that have routinely crippled Washington.

Republicans continue to publicly insist that Democrats must act alone to address the issue, while Democrats have countered that raising the borrowing cap is a shared responsibility given that both political parties have incurred big debts over the last several years.

“Those who believe the debt limit can safely be pushed to the back of the December legislative pileup are misinformed,” said Shai Akabas, the director of economic policy at the Bipartisan Policy Center. “Congress would be flirting with financial disaster if it leaves for the holiday recess without addressing the debt limit.”

Treasury Secretary Janet L. Yellen warned lawmakers in November that the United States could be unable to pay its bills soon after Dec. 15. During testimony before the Senate Banking Committee this week, she underscored the urgency of the matter.

“I cannot overstate how critical it is that Congress address this issue,” Ms. Yellen said. “America must pay its bills on time and in full. If we do not, we will eviscerate our current recovery.”

In September, Ms. Yellen called for the debt limit to be eliminated, explaining that it had become a destructive policy that posed unnecessary risks to the economy. After approaching the first default in American history, Congress in October raised the statutory debt limit by $480 billion, an amount the Treasury Department estimated would allow the government to continue borrowing through early December.

Congressional leaders have been quietly discussing ways to address the debt ceiling, after Republicans warned that they would not help Democrats clear the 60-vote threshold needed to break a Republican filibuster against legislation to raise the borrowing cap.

Senators Chuck Schumer of New York, the majority leader, and Mitch McConnell of Kentucky, the minority leader, have spoken repeatedly in recent weeks about the issue, but they have remained tight-lipped in public about a possible solution.

The debate has been further complicated by former President Donald J. Trump and his continued influence over the Republican Party. He has repeatedly railed at Mr. McConnell and the other Republican senators who backed a procedural vote in October that cleared the way for Democrats to raise the debt limit.

But Mr. McConnell, while pushing for Democrats to raise the borrowing cap without help from his conference, pledged this week that a default would be avoided.

“Let me assure everyone the government will not default, as it never has,” Mr. McConnell said on Tuesday. Pressed further, he added, “We’re having useful discussions about the way forward.”

Cut out of both the $1.9 trillion coronavirus relief package that passed in March and the $2.2 trillion climate, tax and spending plan that Democrats are trying to push through the Senate, Republicans have refused to help Democrats accommodate debt incurred by both parties. They have taken that position even though leaders of both parties signed off on the spending that helped the debt balloon.

Democrats, in turn, have balked at a Republican demand to use a fast-track process known as budget reconciliation to raise the debt limit without Republican votes. Democrats used the process to pass the coronavirus relief package and they are using it again for the climate and tax plan, but they have argued that Republicans should help keep the government from defaulting.

Aides in both parties, while cautioning that a solution has not been agreed to, noted that party leaders had so far refrained from publicly trading blame over the issue.

As a way of navigating around the impasse, some officials have discussed the possibility of handing the authority of raising the debt limit to the administration, while granting Congress the ability to disapprove the decision with just a simple majority.

Some lawmakers, however, may be unwilling to hand that power to the White House or lose a cudgel often used by the minority party to exert pressure, particularly while 60 votes are needed to end a filibuster in the Senate.

Other officials have floated attaching legislation raising the debt limit to the sprawling annual defense policy bill, which is the last major must-pass piece of legislation that lawmakers plan to approve in December.

But it is unclear whether such a plan would be successful: Attaching a debt ceiling increase could jeopardize the Republican votes needed to counter the bloc of liberal Democrats who typically oppose the defense bill in protest of military spending. Representative Kevin McCarthy, Republican of California and the minority leader, warned on Friday that such a maneuver could tank passage of the entire package.

The Bipartisan Policy Center said that there was additional uncertainty surrounding the debt limit this year because of the pandemic and the various economic relief programs that are still ongoing.

Dec. 15 is a particularly important date because the Treasury Department is required to make a $118 billion payment to the Highway Trust Fund. If corporate tax receipts that are due that day come in weak, Treasury could face a cash crunch and the United States could be unable to meet all of its obligations, such as paying out Social Security and funding military paychecks.

The Congressional Budget Office said this week that it expected that Treasury might run out of cash by the end of December if Congress failed to act. The budget office suggested, however, that Treasury might be able to defer some Highway Trust Fund payments that were mandated in the recently passed infrastructure law, potentially staving off a default until sometime in January.

Along with its updated projection, the Bipartisan Policy Center unveiled a new proposal for dealing with the debt limit, although it is unlikely to help lawmakers this time around.

The proposalwhich is being introduced by Representatives Jodey C. Arrington, Republican of Texas, and Scott Peters, Democrat of California, would establish a process giving the president authority to suspend the debt limit through the following fiscal year as long as Congress does not pass a resolution blocking the move within 30 days. The president would then have to offer a debt reduction proposal for Congress to consider separately.