Sen. Bernie Sanders’ (I-VT) presidential campaign has been going after former Vice President Joe Biden for flipping on his decision not to accept donations from Super PACs and therefore opening the door for wealthy supporters to donate unlimited resources to his campaign.

“Here is the truth. Totally honest: Joe Biden’s campaign lacks grassroots enthusiasm. It just does,” Sanders’ campaign wrote in an email to supporters on Friday. “And racing around the country collecting $2,800 checks isn’t enough. He needs bigger checks. Checks with more zeroes at the end.”

“The kind of money most people could never dream of. Because that is the only way he can compete with our 100 percent grassroots funded campaign,” the email continued. “And it might work … This is a big problem. Because honestly, we compete for a lot of the same voters as Joe Biden.”

Biden has been receiving blowback for seemingly reversing his stance not to accept the help from outside Super PACs. After explicitly disavowing the outside financial help earlier in the year, his campaign revealed to NBC News on Thursday that it would, in fact, accept such donations.

Once elected to the presidency, Biden would “push to remove private money from our federal elections,” according to Kate Bedingfield, Biden’s deputy campaign manager. She further noted that he intends to enact a constitutional amendment to overturn Citizens United and “end the era of unbridled spending by Super PACs.”

“Until we have these badly needed reforms, we will see more than a billion dollars in spending by Trump and his allies to re-elect this corrupt president,” Bedingfield said. “And let’s be clear: Donald Trump has decided that the general election has already begun.”

“In this time of crisis in our politics, it is not surprising that those who are dedicated to defeating Donald Trump are organizing in every way permitted by current law to bring an end to his disastrous presidency,” she continued. “Nothing changes unless we defeat Donald Trump.”

The change in policy comes as Biden has been struggling to financially keep up with many of his Democratic Party presidential primary challengers. While Biden generated just under $16 million in the third quarter, his campaign spent more money that it had raised, ending with less than $9 million cash on hand.

The fundraising stands in stark contrast to the $33.7 million and $25.7 million cash reserves that presidential hopefuls Sanders and Sen. Elizabeth Warren (D-MA) reported for the third quarter, respectively. Even second-tier candidates who have been polling significant lower than Biden, such as South Bend, Indiana, Mayor Pete Buttigieg, who has stockpiled cash of $23.4 million, and Sen. Kamala Harris (D-CA), with $10.5 million, are amassing a larger war chest.

“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,” Faiz Shakir, Sanders’ campaign manager said in a statement.

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