It’s now or never for Joe Biden Joe BidenMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE.

After coming in second in Nevada behind Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.), the pressure is on the former vice president to prove that he can win a presidential contest — and become the centrist alternative to the progressive front-runner for the party’s presidential nomination.

To become a real rival to Sanders, aides and allies say he must win the primary this Saturday in South Carolina.

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It would be a big victory for Biden that could restart a presidential campaign that was left sputtering about dismal performances in Iowa’s caucuses and New Hampshire’s primary earlier this month.

Team Biden hopes its fortunes improved with a second-place showing in Nevada’s caucuses. Yet even that result left him far behind Sanders, the winner of that contest.

“He’s gotta f------- win!” one longtime ally said of the South Carolina primary, where polls have for months shown Biden with a lead. “All this other stuff about 'doing well' is horseshit.”

Biden is now facing a serious challenge from Sanders, who has closed the gap in polls of South Carolina — a state Biden aides have labeled his firewall — and is trying to win the state himself, which could push Biden out of the race.

A CBS News–YouGov poll out this weekend showed Biden in the lead with 28 percent support among Democrats and independents who are expecting to vote in the state’s primary. But Sanders has gained on Biden in recent weeks and received 23 percent in the new poll.

If Biden loses the Palmetto State, it will be curtains on his dreams of winning the presidency, and it will be a big problem for others trying to prevent Sanders from winning the nomination as well.

“At this point, anything less than a win makes Sanders’s momentum almost insurmountable,” said Basil Smikle, a former aide to Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE who also served as the executive director of the New York State Democratic Party.

Smikle said that a Sanders win in South Carolina with substantial support from black voters would boost his coalition and do what Iowa did for former President Obama’s 2008 campaign.

“It can brand Sanders’ campaign as multi-generational and multi-racial movement with some ability to siphon some Obama-Trump voters from the president,” he said.

Sanders aside, Biden aides and allies say they feel good about the current state of the race after what they saw as a bounce-back performance in Nevada.

For the first time, Biden did come ahead of the other centrists in the race. Former South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBipartisan praise pours in after Ginsburg's death Bogeymen of the far left deserve a place in any Biden administration Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq MORE had defeated him in New Hampshire and Iowa, while Sen. Amy Klobuchar Amy KlobucharThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill EPA delivers win for ethanol industry angered by waivers to refiners It's time for newspapers to stop endorsing presidential candidates MORE (D-Minn.) finished ahead of the former vice president in New Hampshire.

Biden also got a boost from former New York City Mayor Mike Bloomberg’s disastrous performance in last week’s presidential debate.

“They feel vindicated in a way because they kept saying that Pete Buttigieg wouldn’t do well past the first two states when he hits more diverse terrain and Mike Bloomberg seems to have flamed out fast after the disastrous debate,” said one longtime ally who is regularly in touch with the campaign.

Biden aides and allies also say they will get a boost from the endorsement of House Majority Whip James Clyburn (D-S.C.), who carries tremendous sway in the Palmetto State.

“It’s the best thing that happened to us in weeks,” the second ally said. “By a long shot.”

In an interview on "Face the Nation" on Sunday, Biden said he expects “to do well” in South Carolina.

But the former vice president pushed back on the notion that the state was his firewall.

“You said it’s my firewall, I’ve never said that,” he told host Margaret Brennan. “I’ve said I’m going to do well there.”

The former vice president will spend his time crisscrossing the state this week aiming to solidify his support with black voters, whom he is counting on to be the backbone of the campaign.

Democratic strategist Joel Payne, who served as the director of African American Paid Media on Clinton’s 2016 presidential campaign, said it is critical for Biden to run up the score on Sanders with black voters in South Carolina.

“Not only would that undercut Sanders’s chances at running away with the nomination, but it would harm the other moderates in the race like Buttigieg and Klobuchar as well as build some pressure on them to bow out going into Super Tuesday,” he said.

Buttigieg, Klobuchar, Biden and Bloomberg all appear to be competing for some of the same voters, while Sanders, to his credit and benefit, has been boosted by loyal supporters in each early-contest state.

Democratic strategist Michael Trujillo said that Biden needs a solid debate performance on Tuesday night to build momentum for the weekend vote in South Carolina and for the 14 contests that follow on March 3.

“With people early voting in many Super Tuesday states now, a debate victory is in many ways more important than a major South Carolina win,” Trujillo said. “Biden has to make sure Bernie leaves Tuesday’s debate in a worse shape than how he entered it.”

The second Biden ally said the former vice president agreed that if Biden can walk away with a solid debate performance and doesn’t make any major gaffes, he’ll “be on his way to victory.”

But Payne said Biden can’t just win. He needs a solid victory.

“Meaning it needs to be closer to a 10-point win than a 5-point win,” he said.

The first Biden ally said Biden just needs to win. “Let’s not move the goal post,” the ally said. “A win is a win.”