WASHINGTON — Some Senate Democrats are circulating a letter urging President Joe Biden to prepare to invoke the 14th Amendment to lift the debt ceiling and avert default on his own, without an act of Congress.

The emerging letter, which is still in the works, is a sign of growing progressive dissatisfaction with some aspects of the legislative negotiations, like Republican demands to impose tougher work requirements to receive federal benefits. The letter is being led by Sen. Tina Smith, D-Minn., as well as Sens. Elizabeth Warren, D-Mass., Jeff Merkley, D-Ore., Ed Markey, D-Mass., and Bernie Sanders, I-Vt.

"We write to urgently request that you prepare to exercise your authority under the 14th Amendment of the Constitution, which clearly states: 'the validity of the public debt of the United States...shall not be questioned,'" the senators write in the draft letter, obtained by NBC News. "Using this authority would allow the United States to continue to pay its bills on-time, without delay, preventing a global economic catastrophe."

"While we cannot default on our debt, we also cannot allow the destructive Republican budget to be implemented," they continue.

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The draft letter, which is still being circulated for signatures, comes 15 days before the June 1 deadline set by the Treasury Department for Congress to act or risk breaching the debt limit.

Sanders said in an interview Wednesday, “The message is, in a time of massive income and wealth inequality," Congress cannot "balance the budget on the backs of children and the elderly and working families and force kids to go hungry."

"I mean that is insane. Unacceptable. There is an option out there — it’s the 14th Amendment, it was put into the Constitution for good reason," he said. "The president should implement it.”

Merkley said it’s “important” for Biden to be in a position to reject House Speaker Kevin McCarthy’s demands, which he said amount to an “attack on ordinary working families” and would “unleash fossil fuels on America.”

“I want the president to see that he has the support in the Senate to use the 14th Amendment or other executive powers, including the platinum coin — that he has support to say no to outrageous demands from the radical right that’s trying to terrorize American families,” he said Wednesday.

On Tuesday, Biden met with McCarthy, R-Calif., and other congressional leaders, who later offered signs of hope about the negotiations. But the speaker maintained they are still "far apart."

In the letter, which was first reported by The Washington Post, the Democrats say the GOP's unwillingness to raise taxes on the wealthy and large corporations makes it "seemingly impossible to enact a bipartisan budget deal at this time."

McCarthy flatly ruled out the idea of tax increases on top earners at a news conference Wednesday.

Some experts believe the debt ceiling statute is unconstitutional and cannot prohibit the U.S. from borrowing to pay its debts. But the Biden administration and its predecessors have been reluctant to invoke the 14th Amendment. Then-President Barack Obama's administration looked into it, but his Justice Department never made its opinion on the matter public. Some officials worried that if such a move were to be challenged in court, it could undermine the full faith and credit of the U.S.

Biden said recently he has been “considering” the 14th Amendment but concluded that it would not solve the current stalemate, given that “it would have to be litigated.” But the White House Office of Legal Counsel continues to study the matter, according to a source familiar with the talks, for possible future unilateral use.

Sen. Chris Murphy, D-Conn., voiced skepticism Sunday on NBC's "Meet The Press" when asked if Biden should use the 14th Amendment to continue paying the bills.