Forty-three GOP senators on Saturday signed a letter addressed to Senate Majority Leader Harry ReidHarry ReidDems see surge of new candidates Dems to grind Senate to a halt over ObamaCare repeal fight GOP fires opening attack on Dem reportedly running for Heller's Senate seat 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 CollinsFive things to watch in Senate GOP’s ObamaCare repeal bill McConnell courts GOP centrists for health bill Abortion language may be cut from Senate ObamaCare bill MORE (Maine), Olympia Snowe (Maine), Scott Brown (Mass.) and Lisa MurkowskiLisa MurkowskiFive things to watch in Senate GOP’s ObamaCare repeal bill Overnight Energy: Dakota Access fight enters new phase Overnight Regulation: Labor groups fear rollback of Obama worker protection rule | Trump regs czar advances in Senate | New FCC enforcement chief MORE (Alaska) did not sign on to the letter.

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"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 McConnellMitch McConnellSpecial election sweep boosts Trump agenda Five things to watch in Senate GOP’s ObamaCare repeal bill McConnell courts GOP centrists for health bill 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.