(Adds court ruling)
By Yeganeh Torbati and Jeff Mason
WASHINGTON, April 8 (Reuters) - President Donald Trump's new acting chief of Homeland Security will be under pressure to implement legally dubious solutions to an influx of migrants at the U.S.-Mexico border - policies that his predecessor either could not, or would not, deliver.
Kevin McAleenan, presently commissioner of U.S. Customs and Border Protection (CBP), will be the fourth person to helm the agency under Trump. He takes over as U.S. border officials estimated that 100,000 migrants were apprehended at the southern border in March, the highest level in a decade.
The president, who made immigration a key campaign theme, has grown increasingly frustrated with his officials, even as they have implemented aggressive policies to limit immigration.
Immigration experts say Trump lately has called for policies that would violate U.S. laws, international agreements and court settlements or require U.S. Congress to pass major legislation.
On Friday, he called for Congress to "get rid of the whole asylum system" and get rid of immigration judges, and criticized a long-standing federal court decree mandating certain standards of care for migrant children.
A congressional official familiar with the matter said some in Congress believe Trump forced out Kirstjen Nielsen, who resigned as secretary on Sunday, in part because she was trying to obey laws on treatment of refugees, granting of amnesty and separation of families.
A source close to Nielsen said Trump was convinced to oust her by his senior aide Stephen Miller, an immigration hardliner.
Nielsen did not respond to a request for comment.
It was not immediately clear what strategies McAleenan could implement to achieve Trump's objective of limiting migrant crossings at the southern border, especially as they are expected to reach their yearly peak in the coming months, experts said.
A U.S. judge on Monday halted the administration's policy of sending some asylum seekers back across the border to wait out their cases in Mexico, a policy it said last week it planned to expand. The White House did not respond to a request for comment.
"So much of what the president put out there isn't really legally feasible," said Sarah Pierce, an immigration policy analyst at the Migration Policy Institute, a non-partisan think tank in Washington. "I, like many, and maybe Nielsen herself are kind of puzzled as to what could happen."
A CBP spokesman declined to comment and directed questions to the White House.
McAleenan follows Nielsen and Elaine Duke, who led the DHS on an acting basis after John Kelly, Trump's first DHS secretary, became White House chief of staff in 2017. Trump took office in January that year.
Nielsen oversaw a "zero tolerance" prosecution policy that led to the separation of thousands of parents and children, and launched a policy to return asylum seekers to Mexico until their claims are heard. Both policies garnered legal challenges, and both required extensive implementation by McAleenan and his agency.
Stephen Legomsky, a former chief counsel at U.S. Citizenship and Immigration Services under Democratic President Barack Obama, said McAleenan likely will not have much freedom to pursue policies opposed by Trump or Miller.
"Whoever is put in that position in this administration is going to have a very hard time resisting the philosophy of the White House," Legomsky said.
John Sandweg, former acting director of Immigration and Customs Enforcement (ICE) during the Obama administration, said the Trump administration's focus on deterring migrants at the expense of other policies had hamstrung Nielsen and would likely hobble McAleenan.
"There's nothing we can do that's worse than what people are facing in Central America," he said. "If we're going to work our way through this problem, being tough is not a strategy, it's a soundbite."
White House officials said Trump wanted someone at DHS who would focus on the border as the top priority. McAleenan is seen as having a good relationship with Trump and 20 years of experience, so the president felt he would be a good choice to handle the influx at the border, officials said.
The White House envisions McAleenan working more with Congress, one official said, though the official declined to be specific about policy details.
McAleenan is a rare Trump appointee with cordial relations with Democrats in Congress. After testifying before the Senate Judiciary Committee in December, McAleenan chatted afterwards for close to 15 minutes with Senator Dianne Feinstein and other Democrats on the committee.
"He's not considered to be radioactive," said a congressional Democratic aide on condition of anonymity.
Democratic Representative Joaquin Castro demanded McAleenan resign in December, after a Guatemalan migrant girl died in federal custody and McAleenan failed to report it to Congress within 24 hours, as required.
On Sunday, he said McAleenan's appointment as acting secretary was "deeply disturbing."
Trump further reshuffled the Department of Homeland Security (DHS) on Monday by replacing the director of the Secret Service - which does not have immigration responsibilities - with a career agent.
(Additional reporting by Mark Hosenball and Andy Sullivan in Washington Editing by Kevin Drawbaugh, Susan Thomas and Sonya Hepinstall)
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