Inside the slow-building Biden-Harris relationship

Harris, then two months into her tenure as vice president, was already reckoning with sky-high expectations and intense scrutiny. She wasn’t expecting Biden to ask her to play a similar role on immigration-related diplomacy as he had during the Obama administration, people familiar with the discussions said. Her aides wanted to shield Harris as much as they could from what they knew had become a much more politically toxic issue in recent years.

The episode, which launched the rockiest stretch of the vice president’s tenure, illustrated the unfamiliarity between Harris and Biden at the start of the administration, with Biden looking to his own experience in that role to guide how Harris would fit into his administration. The assignment led to a foiled first foreign trip to Central America for Harris, criticism from immigration advocates and an NBC interview about the border that continues to haunt her.

But over the years—and at times at a cost to Harris’s own political capital—their relationship found firmer footing. Though Harris never broke into Biden’s inner circle of longtime advisers, their relationship became warmer and grew into one of mutual respect, people close to both the president and vice president said. In the aftermath of the president’s halting debate performance, Harris became his strongest public defender, vouching for Biden as other Democrats continued to call on him to step aside.

On Sunday, the president tasked Harris with another job: replacing him on the ticket as the Democratic nominee. This one, she more readily accepted, locking in hundreds of delegates within hours.

The shift in their relationship was gradual, as Harris, the first female and Black vice president, navigated how best to advise the president in private, gain the trust of his allies and help bolster his standing in public. Biden, meanwhile, came to count on Harris’s questioning in meetings and trust her counsel.

During the 2020 campaign, Biden and Harris, who were living in different cities and dealing with pandemic restrictions, didn’t have much time to get to know each other. Harris had worked with the president’s late son Beau Biden, while they were serving as attorneys general in their states, but didn’t know the president personally before the campaign.

“Beau was the kind of guy who inspired people to be a better version of themselves," Harris said during her first joint appearance with Biden as his running mate. “He really was the best of us. And when I would ask him, ‘Where’d this come from?’ he’d always talk about his dad."

Harris entered the White House knowing she had to prove her loyalty to Biden and his top aides. Some of them thought Harris’s ambitions would lead her to overshadow the president and were still skeptical of her after she questioned his busing policy and relationship with segregationist senators during a 2019 Democratic primary debate. Harris terminated her political committees and didn’t staff her top ranks with campaign aides to assuage concerns from the president’s advisers about her bringing what was seen as a messy presidential campaign into the White House.

During her first foreign trip, when the White House asked Harris to deliver a message of deterrence to migrants in Central America, some aides working on the issue warned against it but were ignored. Even though it expressed the administration’s policy, it would come across differently coming from a daughter of immigrants. “Do not come," Harris said in Guatemala, drawing immediate backlash from immigration advocacy groups and Democrats.

In addition to her role on immigration, other jobs that were seen broadly as thankless—including voting rights, which she requested—started stacking up. At the staff level, tensions bubbled between the West Wing and her office. The president’s inner circle was frustrated with media coverage of Harris, including news of staff departures, and complaints from her allies about how she was being used.

But with travel still limited early in the administration, Harris and Biden started to get to know each other one-on-one. The two met for lunch weekly, if their schedules aligned. Biden, at times, popped into Harris’s office while she was on a Zoom with staff, including once to present her with a mug with a drawing of Harris’s face on it that he had bought from a small business in D.C.

Though she had served in the Senate, Harris didn’t come into the White House with as many longstanding relationships on Capitol Hill as Biden did when he was vice president. And while she was in the room with Biden as he pushed lawmakers to pass his legislative agenda, her influence was more on the inclusion of certain policies rather than on the dealmaking.

The vice president pushed for the inclusion of funding to replace lead pipes in the infrastructure bill and investments for historically Black colleges and universities in the early months, people familiar with the discussions said. She also advocated for proposals that didn’t ultimately pass, such as an expanded child tax credit and investments in paid leave, child care and public housing. As Biden faced pressure from progressives to make a decision on student debt cancellation, Harris was one of the major internal backers of the idea and helped Biden in his decision to ultimately support it.

Harris centered her case on canceling student debt to Biden on the relief it would provide Black families and low-income families who never even graduated because of the expenses but still carried student debt. One of Biden’s concerns as he made his decision was that it would be seen as a giveaway to relatively affluent college-educated families.

She also interviewed the finalists for Biden’s Supreme Court nomination and helped the president with his decision.

“I’ve been fortunate to have the advice of the—Vice President Harris—and I mean this sincerely—an exceptional lawyer," Biden said at the White House while announcing his nomination of Judge Ketanji Brown Jackson.

For Harris, trying to establish some internal clout—quietly—often came at the cost of losing external support, people familiar with the issue said.

The vice president, who was still gaining the trust of the president and his advisers, didn’t publicly discuss most of her efforts, even as Democrats who had high expectations for the first female vice president criticized her and the administration for not giving her a more prominent role. They questioned why Harris was constantly positioned behind the president during announcements, with little to do or say.

The Democratic criticism of Harris began to die down in the aftermath of the Supreme Court’s decision to overturn Roe v. Wade. Harris took on a more public role on a policy issue—one she was well-versed in and built her career on. The vice president traveled the country and kept abortion rights—which the president has been uncomfortable talking about—front and center.

Inside the White House, the vice president was frustrated that the administration wasn’t prepared enough for the decision, given they had time to start preparing once Politico published a leaked draft. When Biden left for a trip to Europe to attend global summits the day after the decision, the administration struggled to come up with a detailed plan and messaging strategy. Harris pressed for more specificity, according to people familiar with the matter. The president appreciated Harris’s leadership on abortion rights, and said so publicly, recognizing she was a better messenger for it.

But it was foreign-policy issues that helped the two establish more of a bond.

Harris entered the administration with little foreign-policy experience, especially compared with Biden, who had served as chairman of the Senate Foreign Relations Committee. As Harris started building out her international portfolio and became an active participant in National Security Council meetings, Biden often looked to Harris to ask probing questions, according to people familiar with the discussions. Her back and forths with national security officials helped inform Biden’s thinking.

Since the start of the Israel-Hamas war, Harris has played a key role in urging the administration to articulate more empathy toward Palestinians and focus on a postconflict plan.

Even as she reiterated the White House’s positions on the war, the differences between Biden’s public comments and hers were clear. Harris was more readily critical of Israel and spoke emotionally of the humanitarian crisis in Gaza.

The day after Biden bowed out of the presidential race, Harris walked into her campaign headquarters in Wilmington, Del. Hours earlier, it had been the president’s. Signs with her name now covered some of the walls.

Still self-isolating after testing positive for Covid-19, Biden called in to address campaign staff who now worked for his vice president. “Embrace her," he said.

As Harris began, the president added: “I’m watching you, kid. I love ya."

Write to Tarini Parti at tarini.parti@wsj.com