Pressure is mounting on some candidates to drop out of the Democratic presidential primary race as moderates scramble to unite their faction around a single contender and prevent 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.) from gaining an insurmountable lead in the nominating contest.

The pressure is most acute for 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.) and billionaire activist Tom Steyer Tom SteyerTV ads favored Biden 2-1 in past month Inslee calls Biden climate plan 'perfect for the moment' OVERNIGHT ENERGY: Trump administration finalizes plan to open up Alaska wildlife refuge to drilling | California finalizes fuel efficiency deal with five automakers, undercutting Trump | Democrats use vulnerable GOP senators to get rare win on environment MORE, both of whom posted lackluster showings in the Nevada caucuses on Saturday and face increasingly difficult paths to the nomination.

But other candidates, including former Vice President 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 and former New York City Mayor Mike Bloomberg, also are facing heat.

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Even 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, who notched top finishes in the Iowa caucuses and New Hampshire primary but has struggled to gain traction among minority voters, is facing questions about his prospects as the contest turns to more diverse states, like South Carolina and those that hold their primaries on Super Tuesday.

“I think sooner rather than later, a bunch of these candidates are going to have to understand that they don’t have a viable path to the nomination and need to get behind someone,” said Rufus Gifford, the finance director for former President Obama’s 2012 campaign who is backing Biden.

Biden himself will hear calls to exit the race if he does not win South Carolina’s primary on Saturday.

The onetime front-runner in the race, Biden finished second in Nevada but saw his campaign land in the dumpster with disastrous results in Iowa and New Hampshire.

“For months, Biden’s people have been saying that South Carolina is their firewall,” Brad Bannon, a Democratic strategist, said. “If he doesn’t deliver on Saturday, he’s going to be under tremendous pressure to drop out whether he wants to or not.”

Sen. Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE (D-Mass.) has placed third, fourth and fourth in the first three contests, but believes she is building momentum after a solid debate performance last week. She will need to build more momentum at Tuesday’s debate and then see that turn into votes if she is not to face calls for her to bow out of the race.

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There is growing concern among Democrats opposed to Sanders as their party’s nominee that he could amass a lead so great on Super Tuesday, when roughly a third of pledged delegates are up for grabs, that it would become nearly impossible for any other candidate to catch up to him, let alone surpass him.

The field is also poised to get larger on Super Tuesday, when Bloomberg will appear on primary ballots across the country, raising the possibility of an even-more fractured moderate vote.

None of the candidates are likely to drop out before the South Carolina primary on Saturday, frustrating some Democrats.

“They’re all clogging the drain and no one is accepting any responsibility for it,” one Democratic strategist who has worked on recent presidential campaigns said.

There aren’t any signs that the candidates are facing pressure to drop out from congressional leaders, who have so far been reluctant to inject themselves into the Democratic primary race, given the large number of current lawmakers seeking the party’s nomination.

Klobuchar’s best showing in the nominating contest so far was a third-place finish in New Hampshire. But her sixth-place finish in the Nevada caucuses over the weekend underscored her ongoing struggle to build a diverse coalition of voters, and recent polls suggest that she’s not positioned any better in South Carolina, where a majority of the Democratic electorate is black.

A CBS News–YouGov survey released on Sunday showed her running in a distant sixth place in the Palmetto State with 4 percent support. Among black voters, she notched only 2 percent support.

“Her rationale for staying in the race is pretty shaky,” Bannon said. “She hasn’t really won anything yet to show that she’s competitive.”

Speaking to supporters in Minneapolis on Saturday after the Nevada caucuses, Klobuchar said that she had “exceeded expectations” in the Silver State and would continue on to South Carolina and Super Tuesday, when her home state of Minnesota holds its primary.

Asked by a reporter on Sunday about her rationale for staying in the race, Klobuchar said,“Why would I get out? That’s not even a close call for me.”

Steyer is banking on a top finish in South Carolina’s primary on Saturday to salvage his presidential prospects after finishing near the bottom of the pack in the first three nominating contests. He has spent heavily in the state and is among the top-polling candidates in the lead-up to the primary.

But Steyer, who has yet to win a delegate, faces long odds to win the nomination. Critics say he is effectively siphoning off votes from Biden or Buttigieg.

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“Tom Steyer should absolutely be looking for an exit sign,” Jon Reinish, a Democratic strategist, said. “His money would be far better spent protecting down-ballot candidates and down-ballot incumbents. He’s not going to win. He’s not going to be president.”

Advisers to Bloomberg’s presidential campaign warned in a memo issued last week that Sanders would be “all but impossible to stop” if three candidates — Biden, Buttigieg and Klobuchar — continued to splinter the moderate vote.

“If Biden, Buttigieg, and Klobuchar remain in the race despite having no path to appreciably collecting delegates on Super Tuesday (and beyond), they will propel Sanders to a seemingly insurmountable delegate lead by siphoning votes away from [Bloomberg],” wrote the advisers, Dan Kanninen and Mitch Stewart.

Another memo from Buttigieg’s campaign sent to reporters days later made an identical argument about Bloomberg’s presence in the race.

“If Bloomberg remains in the race despite showing he can not offer a viable alternative to Bernie Sanders, he will propel Sanders to a seemingly insurmountable delegate lead siphoning votes away from Pete, the current leader in delegates,” Buttigieg’s campaign wrote.

Buttigieg now has 24 delegates to Sanders’s 35. Biden has 10 delegates, while Warren has 8 and Klobuchar has 7.

One major Democratic donor said that there’s little incentive for any of the candidates to end their campaigns right now, arguing that they each have arguments for remaining in contention.

“Why would Pete drop out now?” the donor said. “He basically won the first two states and came in third in Nevada. Why would Amy drop out? She can also make the case that her fundraising is good and she’s exceeded expectations. And Bloomberg and Steyer won’t feel any pressure to drop out. Certainly if Biden wins South Carolina, he’ll stay in.”