(CNN)Joe Biden is adopting some of his former Democratic presidential primary rivals' best-known tactics as he seeks to bridge the party's gaps headed into his general election match-up against President Donald Trump.
And he and Buttigieg hosted a virtual "grassroots" fundraiser, a small-dollar event modeled after the events Buttigieg often held, on Friday.
The efforts offer Biden a chance to tap into the popularity and excitement surrounding his former rivals. Part of the aim, a Biden adviser said, is to appeal to the cultural components of past campaigns that are important to those supporters while also maintaining an authentic feeling for Biden and his campaign.
"Our campaign continues to grow stronger because we are adopting some of the smartest, most effective tactics used during the primary, and we're grateful to our friends on other campaigns who have helped us do that," said TJ Ducklo, national press secretary for the Biden campaign. "It's because of this kind of cooperation and unity that we will beat Donald Trump this November."
The calls with Warren and the code borrowed from Buttigieg both align with the image of Biden that his campaign has sought to portray: an empathetic figure who is motivated by the personal connections he makes and stories he hears on the campaign trail.
"These tactics work because they're authentic to Joe Biden," said Lis Smith, the Buttigieg strategist who said she and other former staffers have been in contact with Biden's campaign.
"It doesn't come across as pandering. He's doing it in a way that is authentic to him and that is authentic to his campaign, and that's why I think it's so powerful," she said.
Buttigieg's "Rules of the Road" were the first prominent example of Biden -- who has had to grow his staff for the general election while at home in Delaware, with aides working from their homes, as well -- adopting a former rival's tactics.
Biden and Buttigieg have spoken several times in recent months, and Biden's campaign asked for Buttigieg's feedback and sign-off before making public its "Campaign Code."
"It was really smart to have Pete that involved in this process, because it signals to Pete's supporters that the Biden campaign wasn't just paying Pete's campaign and Pete's supporters lip service," Smith said.
This week Buttigieg emailed his campaign's list to invite them to his first grassroots fundraiser with Biden. It's the sort of event that could bring new online donors into Biden's campaign -- and allow the campaign to hit those donors again and again for contributions.
"Grassroots fundraisers are really important to me. They are based on the idea that the experience of a political fundraiser, often regarded as high-dollar closed-door events in the past, should be equally available to folks chipping in $5, $25 at a time," he said.
This week, Biden's campaign announced the hiring of Julie Chavez Rodriguez, the former co-national political director for the Harris campaign, as a senior adviser focusing on Latino outreach and state operations. The campaign recently beefed up its digital team by adding senior advisers from the campaigns of Harris, O'Rourke and Warren.
Biden's advisers maintain frequent contact with the teams of former opponents. Rob Flaherty, Biden's digital director who is an alum of O'Rourke's campaign, has coordinated digital and social media efforts, and a trio of top advisers -- Cristóbal Alex, Stef Feldman and Symone Sanders -- work with outside groups and former rivals' teams on policy issues.
Biden made his own calls to grassroots donors occasionally during the primary, but he's aimed to make those personalized calls more frequently since he became the presumptive Democratic nominee. Like Warren once did, his team dangles the possibility of a call from Biden in many of its fundraising pitches.
Biden and Warren recently teamed up to call those grassroots donors together.
"I wanted to call to say thank you for contributing to Vice President Biden's campaign. You're one of the people we're counting on," Warren said in a video of the calls. "Today I've got a special guest ... take it away Joe."
"Carroll, this is Joe Biden," the former vice president said. "I was kidding with the senator a moment ago. I said, you know I used to call my contributors, but I never had as many until she endorsed me," said Biden. "I'm counting on her a great deal not just for her endorsement, but for her ideas and her leadership."