As a result, donors to the pro-Biden super PAC, Unite the Country, have specifically asked the group to focus just on countering negative attacks from Trump and others against Biden, according to a donor to the group who gave it those instructions.

For most big Democratic donors right now, donating to super PACs aiding other candidates like Biden and Buttigieg — or throwing in to support New York City's former Mayor Mike Bloomberg — are the only ways they see themselves spending money to try, indirectly, to halt Sanders’ rise.

"Even though there are a lot of people who don’t want Sanders to be the nominee, many view any official anti-Bernie Sanders effort as counterproductive,” said Rufus Gifford, who is helping to do fundraising for Biden and who directed President Barack Obama’s finance operation in 2012. “The more money that goes into an anti-Sanders effort, the more divisive the primary gets and the harder it will be to win in November.”

Sanders has accumulated 21 delegates through the first two 2020 contests, one behind Buttigieg. But he polls well in a series of upcoming states including Nevada, where Democrats vote in caucuses on Saturday and where Sanders is 14 percentage points ahead of the next-highest polling candidate, according to the RealClearPolitics polling average.

Sanders also has built a massive financial advantage over his competitors. Online donors have pumped more than $130 million into his campaign so far — more than any other non-billionaire candidate has brought to the race. That money has allowed Sanders to mount a bigger campaign, running ads in crucial Super Tuesday states like California and Texas before his non-billionaire rivals could afford it. Some candidates, like third-place New Hampshire finisher Amy Klobuchar, may not be able to run ads in large states like Texas at all.

The donors’ conundrum echoes Republicans funders’ agony in 2016 over President Donald Trump. While a minority of Republican funders wanted to act to stop Trump from becoming the nominee, many feared that money spent against Trump would only strengthen his populist, anti-establishment message. Eventually, Our Principles PAC, a super PAC founded by a former Mitt Romney aide, raised $19 million from anti-Trump donors and was among few groups that ultimately emerged up as a face of the “Never Trump” push during the primary.

While most Democratic donors watch silently, two organizations — one focused on Israel — have marshalled money to launch isolated anti-Sanders campaigns.

The Big Tent Project, a 501(c)(4) nonprofit, plans to spend at least $1 million on digital ads in South Carolina spotlighting Sanders’ record on issues including Medicare for All.

As a 501(c)(4), also known as a “dark money” group, the Big Tent Project will not have to reveal its donors. The group’s spokesman, Jonathan Kott, a former aide to Sen. Joe Manchin (D-W.Va.), said Sanders needs more careful vetting in the race, even if some are concerned about leveling damaging attacks on the Democratic Party’s potential standard-bearer.

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“I don’t think his views have been scrutinized at the level of a presidential frontrunner,” Kott said. “There’s a different level of scrutiny that you get when you’re leading in the primary polls.”

Meanwhile, Democratic Majority for Israel, a 501(c)(4) and super PAC focused on pushing the Democratic Party to support Israel, has also aired ads opposing Sanders in Iowa and Nevada, including one spot that featured a voter airing concerns about Sanders’ 2019 heart attack.

Democratic Majority for Israel’s PAC does not draw from the standard stable of big Democratic donors. Several of its major donors have given money to both Democratic and Republicans in the past, disclosures show. Oklahoma-based oil and gas executive Stacy Schusterman, the group’s biggest donor prior to its launch of the anti-Sanders ads in Iowa, has donated to a range of Republican and Democrats in past years, including Senate Majority Leader Mitch McConnell and Mitt Romney’s presidential campaign.

Mark Mellman, who is organizing the Democratic Majority for Israel’s anti-Bernie efforts, said he saw the anti-Sanders ads refocus a conversation about Sanders and electability in Iowa — though after the ad ran, Democratic Majority for Israel staff received “lots of threats from Bernie supporters to our lives and health” on Twitter, Mellman said.

“It certainly set the agenda for the closing discussion in the campaign, which was all about electability. That’s where we wanted the discussion to be,” Mellman said.