“We get phone calls from some of the people who raised for them in California and Texas,” the official said.

The move to identify bundlers — high-dollar donors who have also tapped their personal networks to raise money for the candidate — comes as one of Biden’s top competitors, Pete Buttigieg, recently released his list of bundlers. Buttigieg took the step amid feuding over fundraising transparency between the South Bend, Indiana, mayor and Elizabeth Warren.

Warren, who has shunned private fundraisers and meetings with high-dollar donors, had criticized Buttigieg for meeting with donors behind closed doors and for failing to disclose who was raising big dollars for him.

Buttigieg subsequently opened private fundraisers to reporters, adopting a move Biden had made at the beginning of his campaign.

Buttigieg also released a public list of bundlers featuring more than 100 people who have raised at least $25,000 for the candidate.

Candidates don’t have to disclose bundlers, who are frequently rewarded with ambassadorships and nominations to other administration posts if the candidate wins. But critics have argued that they offer insight into who has the candidate's ear and might hold sway if the person wins. They also help identify potential conflicts of interest.

Biden, in particular, been under pressure to disclose his top fundraisers. Unlike Warren and Bernie Sanders, who both have built a vast small dollar-donor network online, Biden has leaned primarily on private donors.