Joseph R. Biden Jr.’s campaign effectively dropped its longstanding opposition to receiving the assistance of super PACs on Thursday, opening the door for wealthy supporters to spend unlimited amounts of money to try and lift him in the Democratic primary.

The move represents a stark reversal and an implicit acknowledgment of his weakened position in the contest. He entered October with only $9 million in the bank, a fraction of his leading rivals.

Mr. Biden had explicitly denounced support from a super PAC in his 2020 run and had previously said he would not have blessed one if he had run in 2016. As recently as a month ago, when a possible pro-Biden super PAC surfaced, his campaign denounced the effort.

But on Thursday, Kate Bedingfield, his deputy campaign manager, softened that stance, blaming the barrage of ads and attacks from President Trump and his allies and accusing them of interfering in the Democratic primary.