Forty-three GOP senators on Saturday signed a letter addressed to Senate Majority Leader Harry ReidHarry Mason ReidWarner backing 'small carve-out' on filibuster for voting rights Senate hopefuls embrace nuking filibuster Biden fails to break GOP 'fever' 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 CollinsTransit funding, broadband holding up infrastructure deal The Hill's Morning Report - Infrastructure vote fails; partisan feud erupts over Jan. 6 panel Senate falling behind on infrastructure MORE (Maine), Olympia Snowe (Maine), Scott Brown (Mass.) and Lisa MurkowskiLisa Ann MurkowskiWhy Biden's Interior Department isn't shutting down oil and gas Biden signs bill to bolster crime victims fund Bipartisan group says it's still on track after setback on Senate floor 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 McConnellAddison (Mitch) Mitchell McConnellGrassley pressured to run as Democrats set sights on Iowa House Democrats grow frustrated as they feel ignored by Senate Democrats question GOP shift on vaccines 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.