“Donors hedging their bets on Biden because of Bloomberg could be creating a doomsday scenario for Democrats everywhere,” the group’s treasurer, Larry Rasky, wrote. “The Sanders-Warren wing of the Party is ready for the Bloomberg fight. Democrats cannot afford a split Convention.

"If Bernie has more delegates, do you really think the Bros will make way for Mike?," it added, referring to the hard-core Sanders supporters known as "Bernie Bros." "Not to mention that the legacy of the Sanders campaign (such as the Squad) will ravage any chance Center-Left Democrats have of maintaining hard won victories in states from Pennsylvania to California.”

The memo is the most tangible evidence of the anxiety gripping Biden's supporters after his debilitating defeat in Iowa and likely loss in Tuesday's New Hampshire primary. While his supporters are urging patience — they say Biden's strength and his rivals' weakness among African Americans won't be felt until the Feb. 29 South Carolina primary — there are growing fears that his candidacy will implode by then.

It's also an illustration of the dramatic change in tone from Biden and his supporters in the span of a few days. For months, Biden played nice: He wouldn’t throw punches at Sanders and only vaguely referenced Buttigieg. Unite the Country, the pro-Biden super PAC that plowed $6 million into Iowa advertising, followed suit, keeing all of its messaging positive.

The group's newly dire warnings — the subject line of the memo, dated Saturday with the subject line "Avoid the Train Wreck by Uniting For Biden" — come as Biden’s team and top supporters attempt to keep donors from abandoning his campaign.

Biden now is confronted with the reality that as he held his fire in the time he was the frontrunner, two of his competitors surged past him. He’s now scrambling to distinguish himself from both Buttigieg and Sanders, at a time when both are enjoying post-Iowa momentum.

“Biden has been campaigning like he was the frontrunner when he wasn’t,” a senior Sanders campaign advisor said. “He didn’t hit back. He didn’t make his case. He pretended there wasn’t a primary. It was the height of arrogance because he’s arrogant.”

In recent days, the campaign has dramatically shifted its messaging strategy, making Biden available on cable news shows he long shunned in favor of a rose garden strategy. In those interviews, Biden asserted Donald Trump would demolish Sanders because he’s a self-described Democratic socialist.

“If I don't get the nomination and Bernie gets it, I'm going to work like hell for him. But I'll tell you what, it's a bigger uphill climb running as a senator or a congressperson or as a governor on a ticket that calls itself a democratic socialist ticket,” Biden told George Stephanopoulos.

Biden’s campaign created buzz over the weekend with a smoldering digital ad that dismissed Buttigieg’s experience as a small city mayor as amounting to little more than installing decorative lighting and fixing sidewalks.

Moving into New Hampshire, Biden's campaign did not think it would be fighting a multi-front battle. Even after suffering an embarrassing defeat in Iowa, his advisers theorized Elizabeth Warren, who came in third, would be forced to go negative against fellow progressive Sanders to salvage her candidacy, according to a person with knowledge of the campaign’s strategy.

That has not happened. Instead, Warren reiterated in Friday’s New Hampshire debate that she and Sanders are friends and that she’s sticking with her posture of non-aggression.

Biden’s more confrontational tone became necessary after his fourth place finish in Iowa, supporters said.

“Joe being normal Joe is not sufficient in this thing,” said Bill Brodsky, a longtime Biden friend and bundler. “Unfortunately, this is what happens in these primaries, they cannibalize each other. It doesn’t surprise me, but his normal nature is not to be in your face and attacking.”

It’s too early to tell whether the shift in strategy is working.

Virginia Rep. Don Beyer, who was visiting New Hampshire to knock on doors for Buttigieg, said the digital ad might hurt Buttigieg but predicted it won’t help Biden. In the ad, Biden’s accomplishments as vice president are put on a split screen next to Buttiegieg’s as mayor.

“I don’t think it made Joe Biden look good, especially after Pete did the single best job of defending Hunter Biden in the debate,” Beyer said. “It might hurt Pete some but the person it’s most likely to help is Bernie.”

But the sharper-edged messaging was a needed signal to donors that Biden recognized the campaign’s failings in Iowa, advisers and donors said. Unite the Country PAC urged donors to “dig deep right now” to get the former vice president's back.

The super PAC could be running up against its own financial restraints. It poured $6 million of the $7.6 million it had raised on ads and research in Iowa. While it provided nearly $1 million in air cover to Biden in New Hampshire, it faces the daunting challenge of raising funds in the aftermath of two straight losses.

“We feel even more committed to do whatever we can to keep this going,” said Sarah Morgenthau, a Biden donor and fundraiser. “We’re not giving up and he’s not giving up. Sometimes when you experience a setback it just makes you that much more committed to push ahead. That’s what we’re prepared to do.”

Though the super PAC argues that a Sanders vs. Bloomberg would be the ugliest scenario if Biden fails, its memo rips Buttigieg and Klobuchar, too.

"It should also be noted that while Senator Klobuchar had a nice debate performance, her history as a prosecutor has also created problems with our base," it reads. "Beyond all that, Pete’s answer on decriminalization of all drugs is a nonstarter in the general election. And the stature gap on stage is terrifying.”

Both Brodsky and Morgenthau are setting their sights on Nevada and South Carolina, predicting Biden will hit his stride once the primary moves toward states with more diverse electorates.

“Right now, don’t get distracted by Iowa and New Hampshire as far as electoral votes, this is about delegates,” Brodsky said. “I see nobody jumping ship at this point.”

Marc Caputo contributed to this report.