WASHINGTON — President Biden was livid.

He had been in office only two months and there was already a crisis at the southwest border. Thousands of migrant children were jammed into unsanitary Border Patrol stations. Republicans were accusing Mr. Biden of flinging open the borders. And his aides were blaming one another.

Facing his bickering staff in the Oval Office that day in late March 2021, Mr. Biden grew so angry at their attempts to duck responsibility that he erupted.

Who do I need to fire, he demanded, to fix this?

Mr. Biden came into office promising to dismantle what he described as the inhumane immigration policies of President Donald J. Trump. But the episode, recounted by several people who attended or were briefed on the meeting, helps explain why that effort remains incomplete: For much of Mr. Biden’s presidency so far, the White House has been divided by furious debates over how — and whether — to proceed in the face of a surge of migrants crossing the southwest border.

Senior aides have been battling one another over how quickly to roll back the most restrictive policies and what kind of system would best replace them.

Now, Mr. Biden finds himself the target of attacks from all sides: Immigration activists accuse him of failing to prioritize the human rights of millions of immigrants. Conservatives have pointed to surges of migrants at the border as evidence that the president is weak and ineffective. And even some moderate Democrats now fear that lifting Trump-era border restrictions could hurt them politically.

Officials from the Department of Homeland Security expect record numbers of migrants to cross the border this summer, just months ahead of the midterm elections that will determine control of Congress and help shape the arc of Mr. Biden’s presidency for the next two years.

This account of the Biden administration’s handling of the border over the past 15 months is based on interviews with 20 current and former officials, lawmakers and activists, most of whom requested anonymity to discuss private deliberations.

Mr. Biden came into office with high hopes, saying he wanted a system that would allow the United States to determine, in a more compassionate way, which migrants should be allowed to stay in the country. He recruited a team of immigration advocates and others eager to put in place the humane system they had envisioned for years. But the slow pace of change has left some of Mr. Biden’s longtime allies doubting his commitment and wondering whether he is more interested in keeping the highly charged issue from dominating his presidency.

Virtually all of the aides who came on board early in the administration have left the White House, frustrated by what they describe as repeated fights with some of the president’s most senior advisers over whether to lift Trump-era policies. Even some of Mr. Biden’s more enforcement-minded aides have departed.

White House officials did not dispute the internal disagreements over immigration. Vedant Patel, a spokesman for Mr. Biden, said the president understood that changing an outdated system was not “going to happen overnight,” though he said Mr. Biden was “working every day to secure our border and use the power we do have to build a fair, orderly and humane immigration system.”

But the border has been a constant headache for Mr. Biden — one that ballooned into a series of crises even as he tried to stay focused on the pandemic, the economy, Afghanistan and the Russian invasion of Ukraine.

“Our old debates about border security and deterrence and open borders are not capturing the actual policy challenge at hand,” said Andrea R. Flores, who resigned as the director for border management at the National Security Council, disillusioned and frustrated by senior aides and “the opportunities I saw them not take.”

“This is an inflection point,” she said.

Debates and Clashes

Ron Klain issued a warning to his staff last summer.

Mr. Klain, the White House chief of staff, gathered senior aides, including Susan Rice, the president’s domestic policy adviser; Elizabeth Sherwood-Randall, the homeland security adviser; and Amy Pope, the top migration adviser, in the Roosevelt Room. Mr. Klain told them that they needed to make sure the administration was not pandering to people who wanted an immediate end to Trump-era border restrictions, according to two people familiar with his comments.

If they did not find a way to deter soaring illegal crossings at the southwest border, he said, accusations about border chaos would grow worse, anger moderate voters and potentially sink the party during the 2022 midterms.

Mr. Klain was channeling his boss, who had complained to top aides about the intensifying attacks from Republicans characterizing him as an open-borders president, according to a person familiar with Mr. Biden’s comments.

But the source of the president’s frustration was as much from inside his administration as it was from outside. As border crossings increased, disagreements erupted over how quickly to dismantle Mr. Trump’s anti-immigrant policies and what to replace them with.

Record numbers of migrants, including people driven out of their homes in Central America by the economic effects of the pandemic, gangs and natural disasters, surged to the border last summer, in part enticed by Mr. Biden’s promise of a less harsh approach to immigration than that of his predecessor. About 214,000 migrants were taken into custody in July of last year — the first time that many people had been apprehended in a single month in more than two decades.

Mr. Biden has taken a series of actions to reverse his predecessor’s policies. He halted construction of the border wall, created a task force to reunite families separated at the border and reversed Mr. Trump’s ban on considering domestic violence or gang violence as a basis for asylum. He also proposed sweeping legislation to overhaul the nation’s immigration system, though it has stalled in Congress.

But despite those actions, the infighting among the president’s aides was on full display during the Oval Office meeting last March.

Xavier Becerra, the secretary of health and human services, whose department runs shelters for migrant children, said the Department of Homeland Security needed to be more aggressive in turning away older teenagers, which would have changed Mr. Biden’s policy of letting all unaccompanied migrant children into the country. Ms. Rice repeatedly said Mr. Becerra should provide more shelters. Alejandro N. Mayorkas, the secretary of homeland security, said the Department of Health and Human Services needed to move the children more quickly out of overcrowded Border Patrol stations.

Mr. Klain tried to play mediator, to little effect.

For months, aides clashed over an effort intended to speed up consideration of asylum cases at the border by allowing immigration officers to decide the claims rather than overburdened judges. The goal was to streamline a court process that currently takes an average of five years to reach a decision for asylum seekers.

Some of the former immigration advocates in the West Wing, including Ms. Rice’s deputy for immigration, Esther Olavarria, worried that rushing through the new process would limit due process for migrants. Ms. Rice, Mr. Klain and others argued that processing claims faster — and swiftly deporting migrants who fail to win asylum — was an important way to ease the burden on the system and deter illegal crossings.

As the year went on, Mr. Biden grew annoyed by the delays in putting the asylum changes into practice along the border. In meetings on immigration with his top aides, he often asked what resources and funding the team of former advocates and immigration veterans needed for the policy.

The administration did not release the final language for the new policy until last month. And because of staffing and funding issues, the plan will be rolled out slowly and not in time to offer significant help with the expected spike in migrants seeking asylum later this spring.

One of the most fraught debates inside the West Wing over the last year has been what to do about Mr. Trump’s “Remain in Mexico” policy, which forced migrants seeking asylum to wait south of the border until their cases were decided. Human rights advocates assailed the conditions in Mexico, where migrants often stayed in squalid camps where they had no legal representation and were at risk of assault. As a candidate, Mr. Biden had condemned the program. Once in office, he quickly terminated it.

But it was one program that had been effective at keeping some migrants out of border detention facilities. During a meeting last summer convened to discuss options for dealing with the record numbers of migrants at the border, Ms. Sherwood-Randall raised the possibility of restoring the program, with some additional protections for migrants, according to two people familiar with the matter.

That idea horrified immigration advocates inside the administration, who viewed it not only as a breach of Mr. Biden’s campaign pledge, but also as a retreat from the promise of a more humane immigration system.

Mr. Biden, too, appeared uncomfortable with the idea, according to a person who was in the room for the meeting.

The president told his team he would not forbid them to explore whether it was worth restoring the Remain in Mexico policy. But he made it clear that he did not want the idea leaking to the news media — and he did not want his name attached to it. (The Supreme Court would later force the administration to restore the policy.)

Polling by the White House and Democratic groups suggests Mr. Biden is right to be wary of the border issue.

“Republicans are trying to make the case Democrats are the party of chaos,” said Lanae Erickson, a senior vice president at Third Way, a centrist Democratic think tank.

Tensions and Departures

The internal battles over immigration have not been limited to the immigration agencies.

In a meeting last summer, officials from the Centers for Disease Control and Prevention told Mr. Biden’s top aides that it was not clear there was still a public health rationale for keeping the border shut to most migrants, according to three people who attended or were briefed on the discussion.

The Trump administration had seized on Title 42 of the public health code to justify turning away most migrants at the border. When Mr. Biden took office, he said he would not apply the policy to unaccompanied minors, a change from the prior administration. In practice, many families were also let into the United States in spite of the policy.

But by last summer, the coronavirus, including the Delta variant, was already spreading wildly throughout the country. Top C.D.C. officials said it was not clear that keeping out migrants, including asylum seekers, would do much to prevent the spread of a variant already inside the United States.

That was not what some in the White House wanted to hear. Publicly, Mr. Biden and his top aides had always deferred to the C.D.C. when asked about the public health rule, saying it was entirely up to the health agency to decide how long to leave it in place.

But privately, Ms. Rice, Mr. Klain and others were worried that lifting the restriction would invite even more migrants to the southwest border and could be seen as premature if another variant of the virus emerged. White House officials also argued that Title 42 was needed to prevent the spread of the disease along the border.

For the immigration advocates working inside the White House, it was all maddening. They had come to work for Mr. Biden to dismantle the worst policies put in place by Mr. Trump. Now they were being asked to make arguments for continuing them.

Some officials at the C.D.C. resented being blamed for keeping in place what many activists saw as a regressive border policy, according to several people familiar with their complaints.

White House aides also clashed over whether to vaccinate migrants who were let into the United States. Last summer, a plan to administer the coronavirus vaccine was blocked by Ms. Rice and others at the White House, who feared it would encourage more migrants to swarm toward the border seeking a shot.

That was the last thing some advisers — Ms. Rice in particular — wanted, according to several officials who participated in meetings with her on the topic.

Several officials insisted that the frequent and often heated debates about immigration helped them reach better solutions.

“The administration was faced with a thousand cruel policies that were put in place during the last four years,” said Tyler Moran, Mr. Biden’s former senior adviser for migration. “But the administration had its eye on the prize on meaningful policy change.”

However, the fighting inside the administration took a toll on the staff.

In January, Ms. Olavarria, a veteran of decades of immigration debates in Washington and a fierce advocate for migrants, left her job as Ms. Rice’s deputy for immigration. Ms. Moran, who had worked on immigration policy for President Barack Obama and Senator Harry Reid of Nevada, also left the White House. Ms. Flores left in the fall of last year.

Two longtime immigration experts who had agreed to short-term assignments — Ms. Pope, a former Obama administration official, and Roberta S. Jacobson, a former U.S. ambassador to Mexico — left after a few months. An official at the Department of Homeland Security, David Shahoulian, who served as the go-between with the White House, left in September.

The C.D.C. finally announced at the beginning of April that it would lift its public health border restrictions on May 23, around the time of the year when migration typically increases.

But this past week, the issue of Title 42 flared up again as Republicans and some Democrats in Congress held up Covid funding in an effort to protest the administration’s decision to lift the health rule.

Bracing for More

A few days ago in Eagle Pass, Texas, two Nicaraguan women climbed shoeless up the reedy bank of the United States side of the Rio Grande, having just crossed about 250 feet through frigid water. One said she almost drowned.

The women were among more than 8,000 undocumented migrants who crossed the border on Tuesday, many calmly walking up to border officials to turn themselves in. They are the leading edge of what officials believe could be another record-breaking spike this summer.

Crossings are even higher than they were last year, and officials are planning for up to 18,000 migrants a day when the public health rule is lifted — more than twice as many as have been crossing recently.

Homeland Security officials recently released a plan for responding to that spike. An official from the Federal Emergency Management Agency is leading the operation, which aims to swiftly and humanely get migrants through border processing and into immigration detention or to their final destination.

The Border Patrol has been adding large temporary facilities that hold about 500 migrants to use as processing centers. In Eagle Pass and Yuma, Ariz., centers were recently expanded. Another center was built in Laredo, Texas, last year.

Officials say they are prepared.

“This for us, it’s just another variable,” Jason D. Owens, the chief Border Patrol agent in charge of the 245 miles of border in and around Del Rio, Texas, said about the upcoming end to the public health order. “It’s just another thing we have to adapt and adjust to.”

Zolan Kanno-Youngs and Michael D. Shear reported from Washington, and Eileen Sullivan from Eagle Pass, Texas.