At least one group is working on a super PAC to back Biden in the race: A circle of Biden allies including operative Larry Rasky, who worked on Biden's 1988 and 2008 presidential bids. The super PAC has not made any public announcements about when it might become public or start spending money.

In an email, Rasky said, “We intend to fight back against the lies and distortions we’re seeing now from Trump, his allies, the Russians, and the Republican Party."

Like the Biden campaign, Rasky said Biden needs a super PAC in order to fight the Trump campaign.

"No other Democrat has to fight this two-front war. We know Joe Biden is the best candidate to defeat Donald Trump – and so does Donald Trump," Rasky said. "That is why our friend Joe Biden is the target and why we will have his back.”

Millions of dollars in outside contributions may help Biden fend off attacks from Trump, but they will elicit new criticism from Biden's Democratic primary rivals.

Biden has said for months that he did not want help from a super PAC supporting his presidential campaign. Super PACs can raise and spend unlimited money, but big money has become a litmus test in Democratic politics. Many other leading candidates have also disavowed such groups, not wanting to invite attacks from other campaigns or alienate voters who have gravitated to anti-big money messages in recent elections.

Biden has already faced attacks from Democratic primary rivals for relying on bigger money than other candidates to fund his campaign — though he has still been restricted by the $2,800 maximum donation individuals can give to candidates for president in the primary. Bernie Sanders' campaign has persistently criticized Biden for raising money from the nation's financial elite.

"Joe Biden is traveling around the country raising a lot of money for his campaign at fundraising events with billionaires, corporate lobbyists, and Wall Street executives," Sanders campaign manager Faiz Shakir wrote in a fundraising email this summer.

But the former vice president has struggled to raise the small-dollar donations fueling other campaigns, and he finished September with only $9 million in his campaign account — far less than the reserves built by Pete Buttigieg ($23.4 million), Elizabeth Warren ($25.7 million) and Bernie Sanders ($33.7 million). Meanwhile, Biden has been under attack in recent weeks from primary opponents and from Trump, who has spent money on ads hitting the Biden family's business dealings.

Some Biden allies are eager for more campaign cash to rebut the attacks, which an outside super PAC that can easily collect from wealthy contributors backing Biden's presidential bid.

"We’ve got to have enough money to compete against [Trump's] message for the general, and in the primary. That does cost more money and we’ve got to step up," said Dick Harpootlian, a fundraiser for Biden. "A super PAC would be nice."

A super PAC would allow Biden to get additional help from donors who have already given the maximum contribution to his campaign. More than one-third of the $38 million Biden has raised so far comes from donors who already “maxed out” with $2,800 checks and can’t give again in the primary. But a super PAC could get off the ground with large donations from just a handful of those people.

In the campaign statement, Bedingfield emphasized that Biden will continue to "advocate for a constitutional amendment to overturn Citizens United and end the era of unbridled spending by Super PACs," framing the decision as a necessary move to combat Republican spending.

"Donald Trump has decided that the general election has already begun," Bedingfield said. "He and his allies are already spending massive amount of money on paid television and digital advertising to intervene directly in Democratic primaries with the goal of preventing Joe Biden, the opponent that Trump fears most, from becoming the Democratic nominee."

Shortly after Biden's announcement on Thursday, Faiz Shakir, campaign manager for Bernie Sanders, said in a statement that "The former Vice President has been unable to generate grassroots support, and now his campaign is endorsing an effort to buy the primary through a super PAC that can rake in unlimited cash from billionaires and corporations."

"That’s not how we defeat Trump," Shakir said. "It’s a recipe to maintain a corrupt political system which enriches wealthy donors and leaves the working class behind."

Warren also took a jab at Biden — though without naming him — tweeting out late Thursday night: "It’s disappointing that any Democratic candidate would reverse course and endorse the use of unlimited contributions from the wealthy to run against fellow Democrats. A handful of wealthy donors should not be allowed to buy the Democratic nomination. That's not who we are."

Marc Caputo and Matthew Choi contributed to this report.