Forty-three GOP senators on Saturday signed a letter addressed to Senate Majority Leader Harry ReidHarry Mason ReidWATCH: There is no Trump-Russia collusion and the media should stop pushing this The demise of debate in Congress ‘North by Northwest,’ the Carter Page remake MORE (D-Nev.) expressing opposition to his debt-ceiling legislation currently being considered in the upper chamber.

Their unified opposition to the bill leaves Democrats at least three votes short of the 60 needed to a clear cloture and virtually assures its defeat when it comes up for a vote tonight or tomorrow morning.

Only moderate Republican Sens. Susan CollinsSusan Margaret CollinsOvernight Tech: Judge blocks AT&T request for DOJ communications | Facebook VP apologizes for tweets about Mueller probe | Tech wants Treasury to fight EU tax proposal Overnight Regulation: Trump to take steps to ban bump stocks | Trump eases rules on insurance sold outside of ObamaCare | FCC to officially rescind net neutrality Thursday | Obama EPA chief: Reg rollback won't stand FCC to officially rescind net neutrality rules on Thursday MORE (Maine), Olympia Snowe (Maine), Scott Brown (Mass.) and Lisa MurkowskiLisa Ann MurkowskiThe siren of Baton Rouge Interior plan to use drilling funds for new projects met with skepticism The 14 GOP senators who voted against Trump’s immigration framework MORE (Alaska) did not sign on to the letter.

"We are writing to let you know that we will not vote for your $2.4 trillion debt limit amendment which, if enacted, would result in the single largest debt ceiling increase in the history of the United States," reads the letter.

Senate Minority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellLawmakers feel pressure on guns Bipartisan group of House lawmakers urge action on Export-Import Bank nominees Curbelo Dem rival lashes out over immigration failure MORE (R-Ky.) took to the floor moments after the letter was delivered to assure Reid, who was also on the floor, that the proposal had no chance of clearing the chamber.

"It’s not going anywhere," said McConnell. "It will not pass the Senate. It will not pass House."

McConnell also accused Reid of "delaying the inevitable" defeat of the bill and called for an immediate vote. The first procedural vote is currently expected at around 1 a.m. on Sunday. 

Reid responded to the news of the letter, which he apparently received while he was on the floor, by asking McConnell what Republicans would agree to since they oppose his plan.

"What will they vote for?" asked Reid. "Do they have any ideas? Let me know.”

The Reid plan would have President Obama request a $2.4 trillion debt-limit increase in two installments of $1.2 trillion each. The requests would be subject to congressional resolutions of disapproval.