WASHINGTON — President Biden promised to unravel the “moral and national shame” of the immigration policies enacted by President Donald J. Trump. But that was hardly the position Mr. Biden’s lawyers took in a federal courtroom earlier this year.

Appearing in January before a three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit, government lawyers urged the court to let Mr. Biden enforce a restriction imposed by Mr. Trump that allowed migrants to be quickly turned away at the border.

Judge Justin R. Walker, an appointee of Mr. Trump, was confused. The same lawyers had argued weeks earlier that enforcing a different Trump-era border rule would not “align with the administration’s” values. Now, they were essentially saying the opposite.

“What are we to make of this?” Judge Walker asked.

The answer is found in the collision of Mr. Biden’s fervent campaign trail promises to undo Mr. Trump’s harsh immigration policies and the grim reality of trying to manage a surge of migrant crossings amid criticism from Republicans that the president is weak on border security.

The government lawyers in Judge Walker’s courtroom were fighting to uphold a Trump-era public health rule allowing the United States to turn away migrants without providing them an opportunity to ask for asylum.

They have sought to defend the Biden administration — and former Trump administration officials — against lawsuits from parents who were separated from their children at the border, even after Mr. Biden called the separations “criminal.” And winners of a visa lottery, including those increasingly at risk in Ukraine, were surprised to see federal lawyers continue to delay the processing of their green cards.

The gulf between Mr. Biden’s words and his government’s legal arguments is testing the patience of some of his supporters, including top Democrats in Congress. They say the administration is not only moving too slowly on promised reforms, but also is far too willing to use — and defend — Trump-era policies in the meantime.

“The only way I know if I’m reading a Biden or Trump administration brief is by looking at the signature block,” said Lee Gelernt, a lawyer with the American Civil Liberties Union. “There’s simply no daylight on the legal positions.”

The tension has also resonated inside the White House, where senior officials have been anxious that unwinding the Trump-era border restrictions would open the United States to an increase in illegal crossings at the southern border and fuel Republican attacks that Mr. Biden is too lenient on illegal immigration.

Mr. Gelernt cautioned that the similarities between the approaches of the Biden and Trump administrations extended only to Mr. Biden’s defense of certain immigration restrictions. Mr. Gelernt acknowledged that the Biden administration had worked to unwind other policies and help unite minors who were separated from their parents.

“Are Biden and Trump the same on immigration? No,” said Mr. Gelernt, who is representing families who were separated at the southern border in 2018. “Has Biden lived up to his campaign rhetoric? Also no.”

Mr. Biden has indeed taken steps to roll back much of his predecessor’s agenda on immigration, including sweeping bans on Muslim-majority countries and a rule allowing officials to deny green cards to immigrants in need of public assistance.

He has taken nearly 300 executive actions on immigration, according to the Migration Policy Institute. Nearly 90 of them were to at least begin rolling back Trump administration policies, most of them technical rules that typically went unnoticed by the public.

The administration has also allowed minors to cross the border. This weekend, the Centers for Disease Control and Prevention exempted children and teenagers from the Trump-era border rule, which justifies rapidly turning away migrants as a public health measure, minutes before a court order would have forced Mr. Biden to apply the rule to the minors.

Weeks before he came into office, Mr. Biden warned that he would need time to undo Trump-era rules, in part because he said unwinding them immediately could encourage migrants to journey to the border. While the administration declined to comment on continuing litigation, Vedant Patel, a White House spokesman, said, “We all agree that our immigration system is outdated and in bad need of reform.”

But, he said, “Making the necessary changes isn’t going to happen overnight.”

Still, many people, including some caught up in the Russian invasion of Ukraine, say time is running out for them.

Iryna Bohdan, a 50-year-old Ukrainian, won a green card lottery in May 2019 through the diversity visa program, which prioritizes countries with low levels of immigration to the United States. But she was barred from entering the United States because of the Trump administration’s pandemic restrictions.

She got her hopes up when Mr. Biden took office because he had celebrated the visa lottery during his campaign and even proposed expanding the program by about 25,000 visas. Last fall, two judges ordered the Biden administration to process the backlogged visas this year.

But Justice Department lawyers have appealed the court orders, saying the government still lacks the resources to process the visa applicants without delaying future winners of the lottery.

This month, the lawyers also argued that the orders would undermine the president’s ability to impose future travel restrictions on other visa applicants, including Russian officials hit with sanctions by the United States for invading Ukraine.

“We don’t know what to do,” said Ms. Bohdan, who fled to Poland this month with her 14-year-old twin sons. Speaking through a translator, she said her family had been visiting relatives in northeast Ukraine before the war began and did not have time to go home to get clothes, belongings or the animals they care for at their veterinary clinic.

Vitali Demchenko, who won the visa lottery more than two years ago, said he now stayed up at night in his home in Ternopil, in western Ukraine, listening for sirens warning of a Russian attack, just in case his family needs to hide in a bomb shelter.

“We always have our suitcases ready and our clothes ready to go,” Mr. Demchenko said from his home before his wife advised him to cut off the phone interview. They needed to return to the bomb shelter, he said.

“Everything is upside down,” he said. “I’m in shock.”

Lawyers representing families who were separated at the border under Mr. Trump said they, too, were surprised by Mr. Biden’s approach.

While Mr. Biden formed a task force last year to help unite minors who were separated from their parents under Mr. Trump’s “zero tolerance” policy, the government walked away from settlement negotiations late last year for the families of some of the 5,500 children.

After it was revealed that the government was considering $450,000 payouts to families, which prompted a backlash from Republicans and conservative outlets, Mr. Biden told reporters that the payouts were “not going to happen.”

The president later said the families deserved some form of compensation. But the administration pulled out of the negotiations in December and has sought to get the courts to dismiss families’ lawsuits against not just the United States, but also the Trump administration officials involved with putting the policy in place.

Lynden Melmed, who served as chief counsel for the government’s legal immigration agency during President George W. Bush’s administration, said there had for years been a break between a presidential candidate’s platform and the approach taken by his legal team once in office.

Lawyers for the federal government are not just focused on campaign promises, but on how one policy argument could undermine legal arguments across the government.

“It’s easy to write talking points,” Mr. Melmed said. “It’s much more difficult to run an agency.”

Immigration lawyers and other Democrats, including Senator Chuck Schumer, the majority leader, have been increasingly outspoken about Mr. Biden’s continued use of Title 42, the emergency public health order that the government has said allowed border agents to turn away migrants at the nation’s borders to prevent the spread of the coronavirus pandemic.

During his presidential campaign, Mr. Biden did not specifically say that he would rescind the rule, but he committed to restoring the asylum process — an opportunity denied to many migrants subject to the public health order. Vice President Kamala Harris went further when she was a senator, criticizing the Trump administration for misusing a limited public health authority as a sweeping immigration tool.

Critics, including multiple former Biden administration officials, say the government is using the rule as an easy way to quickly remove migrants who are gathering at the southern border — and to fend off Republican attacks.

A federal appeals court dealt a blow to the rule this month when it said the administration could no longer use the rule to expel migrant families to places where they would face persecution or torture.

Even though the court allowed Mr. Biden to keep the rule in place, some immigration advocates said the decision could be the beginning of the end of the border rule, and the administration has drafted plans to lift it by April, according to officials.

During a hearing in January, a lawyer for the government, Sharon Swingle, said using Title 42 to keep asylum seekers south of the border was necessary to reduce the spread of the coronavirus in border detention facilities.

Judge Walker reminded the Ms. Swingle that the Biden administration had previously argued that forcing migrants back to Mexico while they await their asylum cases would not “align with the administration’s values” because of the risk of violence and sexual assault from cartel members south of the border.

But Ms. Swingle once again pointed to the public health emergency as reason to continue using Mr. Trump’s border restriction.

“I don’t believe,” she said, “there is a contradiction here.”

Eileen Sullivan contributed reporting.