This year's most far-reaching immigration case is likely to decide if immigration agents in Los Angeles are free to stop, question and arrest Latinos they suspect are here illegally.

President Trump promised the "largest mass deportation operation" in American history, and he chose to begin aggressive street sweeps in Los Angeles in early June.

The Greater Los Angeles area is "ground zero for the effects of the border crisis," his lawyers told the Supreme Court this month. "Nearly 2 million illegal aliens — out of an area population of 20 million — are there unlawfully, encouraged by sanctuary-city policies and local officials' avowed aim to thwart federal enforcement efforts."

The "vast majority of illegal aliens in the (Central) District (of California) come from Mexico or Central America and many only speak Spanish," they added.

Their fast-track appeal urged the justices to confirm that immigration agents have "reasonable suspicion" to stop and question Latinos who work in businesses or occupations that draw many undocumented workers.

No one questions that U.S. immigration agents may arrest migrants with criminal records or a final order of removal. But Trump administration lawyers say agents also have the authority to stop and question — and sometimes handcuff and arrest — otherwise law-abiding Latinos who have lived and worked here for years.

They could do so based not on evidence that the particular person lacks legal status but on the assumption that they look and work like others who are here illegally.

"Reasonable suspicion is a low bar — well below probable cause," administration lawyers said. "Apparent ethnicity can be a factor supporting reasonable suspicion," they added, noting that this standard assumes "lawful stops of innocent people may occur."

If the court rules for Trump, it "could be enormously consequential" in Los Angeles and nationwide, said UCLA law professor Ahilan Arulanantham, co-director of the Center for Immigration Law & Policy. "The government would read this as giving immigration enforcement agents a license to interrogate and detain people without individualized suspicion. It would likely set a pattern that could be used in other parts of the country."

In their response to the appeal, immigrant rights advocates said the court should not "bless a regime that could ensnare in an immigration dragnet the millions of people ... who are U.S. citizens or otherwise legally entitled to be in this country and are Latino, speak Spanish" and work in construction, food services or agriculture and can be seen at bus stops, car washes or retail parking lots.

The case now before the high court began June 18, when Pedro Vasquez Perdomo and two other Pasadena residents were arrested at a bus stop where they were waiting to be picked up for a job. They said heavily armed men wearing masks grabbed them, handcuffed them and put them in a car and drove to a detention center.

If "felt like a kidnapping," Vasquez Perdomo said.

The plaintiffs include people who were handcuffed, arrested and taken to holding facilities even though they were U.S. citizens.

They joined a lawsuit with unions and immigrants rights groups as well as others who said they were confronted with masked agents who shouted commands and, in some instances, pushed them to the ground.

However, the suit quickly focused not on the aggressive and sometimes violent manner of the detentions, but on the legality of the stops.

U.S. District Judge Maame Ewusi-Mensah Frimpong said the detentions appeared to violate the 4th Amendment's ban on unreasonable searches and seizures.

It is "illegal to conduct roving patrols which identify people based on race alone, aggressively question them, and then detain them without a warrant, without their consent, and without reasonable suspicion that they are without status," she said on July 11.

The crucial phrase is "reasonable suspicion."

For decades, the Supreme Court has said police officers and federal agents may stop and briefly question persons if they see something that gives them reason to suspect a violation of the law. This is why, for example, an officer may pull over a motorist whose car has swerved on the highway.

But it was not clear that U.S. immigration agents can claim they have reasonable suspicion to stop and question persons based on their appearance if they are sitting at a bus stop in Pasadena, working at a car wash or standing with others outside a Home Depot.

Frimpong did not forbid agents from stopping and questioning persons who may be here illegally, but she put limits on their authority.

She said agents may not stop persons based "solely" on four factors: their race or apparent ethnicity, the fact they speak Spanish, the type of work they do, or their location such as a day labor pickup site or a car wash.

On Aug. 1, the 9th U.S. Circuit Court of Appeals refused to lift the judge's temporary restraining order. The four factors "describe only a broad profile that does not supply the reasonable suspicion to justify a detentive stop," the judges said by a 3-0 vote.

The district judge's order applies in the Central District of California, which includes Los Angeles and Orange counties as well as Riverside, San Bernardino, Ventura, Santa Barbara and San Luis Obispo.

The 9th Circuit said those seven counties have an estimated population of 19,233,598, of whom 47% or 9,096,334 identify as "Hispanic or Latino."

Like Frimpong, the three appellate judges were Democratic appointees.

A week later, Trump administration lawyers sent an emergency appeal to the Supreme Court in Noem vs. Perdomo. They said the judge's order was impeding the president's effort to enforce the immigration laws.

They urged the court to set aside the judge's order and to clear the way for agents to make stops if they suspect the person may be in the country illegally.

Agents do not need evidence of a legal violation, they said. Moreover, the demographics of Los Angeles alone supplies them with reasonable suspicion.

"All of this reflects common sense: the reasonable-suspicion threshold is low, and the number of people who are illegally present and subject to detention and removal under the immigration laws in the (the seven-county area of Southern California) is extraordinarily high," wrote Solicitor Gen. D. John Sauer. "The high prevalence of illegal aliens should enable agents to stop a relatively broad range of individuals."

He said the government is not "extolling racial profiling," but "apparent ethnicity can be relevant to reasonable suspicion, especially in immigration enforcement."

In the past, the court has said police can make stops based on the "totality of the circumstances" or the full picture. That should help the administration because agents can point to the large number of undocumented workers at certain businesses.

But past decisions have also said officers need some reason to suspect a specific individual may be violating the law.

The Supreme Court could act at any time, but it may also be several weeks before an order is issued. The decision may come with little or no explanation.

In recent weeks, the court's conservatives have regularly sided with Trump and against federal district judges who have stood in his way. The terse decisions have been often followed by an angry and lengthy dissent from the three liberals.

Immigration rights advocates said the court should not uphold "an extraordinarily expansive dragnet, placing millions of law-abiding people at imminent risk of detention by federal agents."

They said the daily patrols "have cast a pall over the district, where millions meet the government's broad demographic profile and therefore reasonably fear that they may be caught up in the government's dragnet, and perhaps spirited away from their families on a long-term basis, any time they venture outside their own homes."

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