Washington (CNN) Some moderate Democrats alarmed by Bernie Sanders ' rapid ascension as their party's presidential front-runner are scrambling for ways to halt his candidacy ahead of Super Tuesday's coast-to-coast contests .

But, so far, many of the party's major donors have remained on the sidelines -- holding out hope that another consensus, moderate candidate could emerge from the still-crowded field after the next round of voting and wary of triggering a backlash from Sanders' devoted supporter base. Meanwhile, supporters of former Vice President Joe Biden , whose own campaign has floundered since primary voters started heading to the polls,are touting a super PAC supporting his candidacy as the best vehicle for wealthy Democrats seeking an alternative to the Vermont senator.

The stakes are high: On Saturday, voters in South Carolina head to the polls in the first-in-the South primary. Days later, the nomination fight barrels through 14 Super Tuesday states -- including delegate-rich California and Texas. Sanders, who has campaigned in those states in recent days, could rack up enough wins by mid-March to build an insurmountable delegate lead.

"There is anxiety about whether (Sanders) can win and whether he will be a drag down-ballot," said Gara LaMarche, the president of the Democracy Alliance, a coalition of Democratic political donors co-founded by financier George Soros. But, he said, "there is no sign that I pick up among the donors that I deal with of any active 'Stop Bernie' measure."

The party's contributors have not coalesced around an anti-Sanders strategy, in part, because of the fluid nature of a race that saw Biden in the pole position for most of 2019, said David Brock, a veteran Democratic operative who serves as chairman of an anti-Trump super PAC. Over the course of the campaign, other candidates, such as Massachusetts Sen. Elizabeth Warren, surged in popularity. Former New York mayor Michael Bloomberg, who has spent nearly half a billion dollars on the race, was viewed as a potential moderate foil to Sanders until a shaky performance in his debut on the debate stage last week.

"It's only been in the last week or so where a real panic has set in that Sanders could be the nominee," Brock said.

"The donor class is, as usual, way behind the eight ball," said one Democratic strategist who opposes Sanders. "They didn't really perceive a threat from Sanders at the beginning, when it was clearly there. And now they see it, and they think it's too late."

Bernard Schwartz, a Democratic megadonor, told CNBC this week that he has been trying to reach out to Democratic leaders in Congress to encourage them to endorse another candidate to thwart Sanders. He did not immediately respond to a CNN interview request.

But, despite the concerns about Sanders' hard-left positions, some donors have begun to view his candidacy in a new light, arguing that the self-described democratic socialist might have a narrow path to a general election victory -- given his fundraising strength, loyal supporter base and dogged advocacy on kitchen-table issues, such as health care, that have animated the Democratic electorate.

"People are taking another look," LaMarche said. "There's nothing like winning to get people to reconsider their views."

In a Medium post this week , entitled "Don't Panic," Democratic donor and activist Dimitri Mehlhorn wrote that "while he may not be our best bet, Bernie is a viable general election candidate."

Among his attributes, Mehlhorn wrote: Sanders' ability to connect with the white working-class voters who skewed heavily toward Trump in 2016, his popularity with young voters and his performance in head-to-head polling matchups against Trump.

Too little, too late?

Apart from a digital video this week that slams Sanders' past record on guns, Bloomberg has not trained his considerable firepower on the Vermont senator on the airwaves so far. And Democratic strategists warn Sanders is racing to the Super Tuesday states with his decades-long record in public life largely unvetted.

Brock, a longtime ally of Hillary Clinton who conducted opposition research into Sanders during the 2016 presidential race, said many voters still know little about Sanders, his policies and past positions.

"It's pretty dangerous that he's unvetted at this moment when he's poised to possibly wrap this up," he said.

The job of taking Sanders to task should fall to Bloomberg, Jim Messina, who managed President Barack Obama's reelection campaign in 2012, argued on CNN this week.

"Forty percent of voters will vote in the next week. People are making up their minds every day, and somebody's got to point out Bernie Sanders' flaws," he said. Bloomberg has "the biggest checkbook."

So far, the two anti-Sanders groups to emerge in recent months have not spent substantial sums in a presidential campaign where advertising already is nearing the $1 billion mark. One, the Big Tent Project Fund, has reported spending about $570,000 on anti-Sanders activity, according to its filings with the Federal Election Commission.

Another group that had been active ahead of the Iowa caucuses, a political committee aligned with the Democratic Majority for Israel, spent roughly $702,000 on its advertising targeting Sanders, according Kantar Media's Campaign Media Analysis Group (CMAG).

Money leader

Biden is counting on a win in South Carolina's primary Saturday to revive his candidacy.

African Americans are expected to make up about 60% of Saturday's Democratic electorate, based on past turnout. And Biden got a boost Wednesday with an endorsement there from Rep. Jim Clyburn, the No. 3 Democrat in the US House of Representatives who is the nation's highest-ranking African American in Congress and a respected figure in the state's black community.

But a few days later, the former vice president faces the 14 Super Tuesday states -- and questions about whether he has the financial resources to mount a competitive campaign across the country.

This week, his campaign announced it would run its first ads in Super Tuesday states, which it described as a "six-figure buy" in eight states. By comparison, Bloomberg already has spent more than $160 million on advertising and future reservations in all 14 states that will vote March 3, CMAG's tally shows.

And Sanders has spent nearly $15 million to advertise in Super Tuesday states.

Biden entered February with just $7.1 million in cash reserves, according to his campaign's most recent filings with the FEC. By comparison, Sanders had nearly $17 million remaining in his campaign account. Warren was in the most precarious position among the top-tier Democratic candidates, with just $2.3 million remaining her account, including $400,000 she drew from a line of credit her campaign secured in January.

(Warren's aides say she raised an additional $14 million in the days leading up to Nevada's caucuses.)

Apart from the two billionaires seeking the nomination, Sanders has built a massive fundraising advantage over the rest of the field, driven largely small-dollar donors who contribute online. In all, he's taken in more than $120 million in contributions during the election cycle.

And Sanders' army of grassroots donors have demonstrated their willingness to rally to his aid when he's under attack. Earlier this year, the Democratic Majority for Israel's PAC released its ads in Iowa, questioning Sanders' electability and his health following his heart attack last fall.

His campaign swiftly denounced the effort in an email to supporters and on social media. Within 24 hours, Sanders' aides said they had raised $1.3 million.

Biden or Bloomberg

Outside of Capitol Hill , Wall Street may be the epicenter of anxiety over Sanders' march to the nomination.

He has said billionaires should not exist and wants to break up big banks -- a step he argues is necessary to ward against a repeat of the 2008 financial crisis. His plans include introducing taxes on stock trades to help fund his free college proposals and imposing new taxes on income and accumulated wealth . His "Medicare for All" plan, if enacted, would end private insurance and upend the health care industry that represents nearly 20% of the nation's economic activity.

An example of the intense animosity: Former Goldman Sachs CEO Lloyd Blankfein recently said on Twitter that Sanders is "just as polarizing as Trump AND he'll ruin our economy."

Sanders' retort: "I welcome the hatred of crooks who destroyed our economy."

Venture capitalist Alan Patricof, a longtime Democratic donor and fundraiser backing Biden, said both Sanders and Warren hold views that are too extreme for him and many in his circle of contributors, but he sees no signs of a coordinated push among donors to stop Sanders.

"I'm hoping Joe is going to prevail, and if he doesn't, I hope it's Mike (Bloomberg)," he said. "They're safe. They're smart. They understand the issues, and they know how to get things done."

Biden's supporters are urging Democrats to open their wallets to a pro-Biden super PAC, called Unite the Country, as the best way to influence outcome of the race. The group had raised a little more than $7.9 million through the end of December, according to its more recent filing with federal election officials.

Amanda Loveday, a spokeswoman for the super PAC said fundraising has improved in recent days, following what she called Biden's "strong and steady" performance during Tuesday night's televised debate in Charleston, South Carolina.

"A number of people came to us today and said, 'We're ready'" to give, Loveday told CNN on Wednesday. She would not say how much the group has raised. Unite the Country now is working to reinforce Clyburn's endorsement with voters ahead of Saturday's primary with digital and radio ads and mailers.

Socialist label

As establishment Democrats wring their hands, Republicans say they are ready to capitalize on Sanders' rise. The Democratic majority in the House is built on the more than three dozen seats the party flipped from Republican control in the 2018 midterm elections.

If Sanders becomes the nominee, the GOP's longstanding playbook to tie Democrats to socialism becomes even "more effective" in these Republican-leaning and swing districts, said Calvin Moore, a spokesman for the Congressional Leadership Fund, the leading Republican super PAC focused on House races.

The voters who tossed out Republican incumbents in 2018 may have sought a "reversion to the mean," he said. But "they didn't sign up for the type of wholesale amending of their lives that socialism would bring."

Despite their alarm, Democratic donors and strategists interviewed this week say the party has little choice but to present a united front if Sanders secures the nomination.

"My understanding is that 90% of Bernie's voters voted for Hillary, but 10% didn't," former Pennsylvania governor Ed Rendell told CNN, referring to the 2016 general election. "That 10% would have been enough to easily carry Pennsylvania, Michigan and Wisconsin. That can't happen again."

Rendell backs Biden, but he said: "If Bernie is our nominee, we must bite our lip and get in there and do everything we can for him."