The US Senate prepared to take its most consequential step in years with the procedural vote Thursday on President Donald Trump's Supreme Court pick.
Democrats made last-minute pleas against changing rules to ram through the nomination of Neil Gorsuch.
But Republicans are keen to get Gorsuch elevated to the nation's highest court to fill the seat vacated by justice Antonin Scalia, who died early last year.
Most Democrats -- spiteful toward Trump, suspicious of his nominee, and livid with Republican leaders for not proceeding with president Barack Obama's Supreme Court nominee last year -- are desperate to block him.
Their acrimonious battle boils down to two critical votes Thursday: the first on a blocking technique known as a filibuster against Gorsuch by Democrats, and a subsequent vote with Republican support to change Senate rules -- the "nuclear option" -- in order to get Trump's nominee confirmed.
The move would bulldoze century-old Senate traditions and potentially do irreparable damage to the deliberative body.
"Step back from the brink," top Senate Democrat Chuck Schumer implored Majority Leader Mitch McConnell.
"If the majority leader breaks the rules tomorrow, and that would be his choice, he would be forever unwinding that important principle, erasing the last shred of bipartisanship in the Senate confirmation process."
Republicans and Democrats have clashed bitterly over Gorsuch, with no signs of averting the standoff.
Opposition Democrats insist they have the necessary votes for a successful filibuster.
Sixty votes are needed to overcome a filibuster and end debate in the 100-seat Senate. Since Republicans hold 52 seats, they would need eight Democrats to break ranks and support Gorsuch.
Only four have done so to date.
Never in US history has a partisan filibuster blocked a Supreme Court nominee.
But with that appearing all but certain Thursday, McConnell was set to change Senate rules in order to advance the nomination -- and all subsequent Supreme Court nominees -- by simple majority vote.
A confirmation vote follows on Friday, after which Congress shutters for a two-week recess.
The magnitude of the choice was beginning to dawn on senators in the hours before the vote.
"We are in a terrible place," a somber Senator John McCain, the 2008 Republican presidential nominee, said on the floor.
"What we are poised to do at the end of this week will have tremendous consequences and I fear that someday we will regret what we're about to do."
With just a simple majority needed on all judicial nominations, compromise candidates will be a thing of the past, McCain said, and "we will see more and more nominees from the extremes of both left and right."
- 'More extreme, more divided' -
Thursday's is the most consequential rules change in the Senate since Democrats controlled the chamber.
Frustrated by Republican obstruction of dozens of Obama's lower-court picks, Democratic leaders controversially employed the nuclear option in 2013.
They lowered the threshold for judge nominations to 51 from 60 -- but they kept the three-fifths threshold in place for Supreme Court nominees.
This time it was Democrats pleading against new changes, for a second straight marathon night on the Senate floor.
Senator Jeff Merkley delivered one of the longer speeches in Senate history by holding the floor for more than 15 hours Tuesday into Wednesday morning to oppose Gorsuch.
Still the positions appeared to be firmly set, with each side accusing the other of steering the Senate onto its collision course.
"Democrats would filibuster Ruth Bader Ginsburg if President Trump nominated her," an exasperated McConnell told the Senate, referring to the iconic liberal Supreme Court justice.
"We all know why. Democrats are bowing to hard-left special interests that can't get over the results of the election."
Senator Brian Schatz was among several Democrats urging lawmakers to reconsider rushing to diminish the Senate's own power at a time of great polarization over the country's president.
Republicans, he said, should tell McConnell to postpone the vote until after the two-week recess.
"Give us some time to find another way to do this. Because otherwise, you will make the Supreme Court, and this place, this country, more extreme and more divided," Schatz said.
"You will answer this difficult moment in history by weakening one of the last bastions of bipartisanship," he added. "And I believe you will regret it."