The swell of cash came from 182,000 donations, with $28 being the average amount given, according to figures provided to The Associated Press by the campaign, which did not include money that Biden raised through big-dollar fundraisers. It comes after his internet fundraising operation stumbled over the summer, leading critics to suggest he lacked grassroots support.
"All of the Trump attacks have started to catalyze. More people understand what is at stake," deputy campaign manager Pete Kavanaugh said in an interview. "People out there are seeing Joe Biden getting attacked day after day. They understand he needs to fight back."
The fundraising boost comes amid growing anxiety over Biden's campaign from would-be allies in the Democratic establishment, who have fretted about his prospects following underwhelming debate performances, middling fundraising success and withering attacks from rivals in his own party and from Trump.
Biden reported raising a less-than stellar $15 million during the third quarter, which ended in September. Though his support has shown no sign of cratering in public opinion polls, the amount was millions less than what Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts took in at the time.
Biden also spent more than he took in during that period and reported having only $9 million in cash on hand at the end of September. His campaign says that's partially due to start-up costs to build out his online operation.
"We knew we were going to have to spend money and we were comfortable with that," Kavanaugh said. "You always want more money. But we believe we had made the right decision. We'd rather end with $9 million on hand than not make those investments."
Over the summer the campaign scaled back spending on fundraising advertisements on social media platforms like Facebook. Campaign aides say this was because a crush of candidates trying to reach grassroots fundraising thresholds to qualify for the debate stage made the cost of the social media advertisements spike and they viewed it as a poor return on investment.
His campaign says the $5.3 million raised online in one month demonstrates that his fundraising prospects are now improving, and he will have the resources needed to compete in Iowa, which will hold the first Democratic primary contest in early February.
Still, it took Biden becoming the target of attacks from Trump and his allies for the online fundraising to really take off. The boost comes as House Democrats are moving forward with an impeachment inquiry that is probing Trump and his associates' efforts to strong-arm Ukrainian officials to provide damaging information on Biden.
Trump has sought, without evidence, to implicate Biden and his son Hunter in the kind of corruption that has long plagued Ukraine. Hunter Biden served on the board of a Ukrainian gas company at the same time his father was leading U.S. diplomatic dealings with Kyiv under President Barack Obama. Though the timing raised concerns among anti-corruption advocates, there has been no evidence of wrongdoing by either the former vice president or his son.
But that hasn't stopped Trump, who has attacked Biden during public appearances and in political ads. This week, Biden supporters created an outside spending group, Unite the Country, to help fight back. Though Biden initially opposed getting help from such a group, which can raise unlimited sums so long as it does not coordinate with his campaign, he reversed course after the attacks against him ramped up.
"Right around the time the Ukraine stuff broke wide open and (Trump) made clear that he and the (Republican National Committee) were going to spend money attacking Biden — that's when you see this start to change," Kavanaugh said. "It got people to snap to attention."