The stay will remain in place until the Supreme Court considers whether to add the case to its docket. Such review is likely and is virtually certain to allow Trump to continue the policy — often called “remain in Mexico” but formally dubbed the "Migrant Protection Protocols" — through the November election.

White House press secretary Stephanie Grisham hailed the high court‘s action as “a major victory.“

“By allowing the Migrant Protection Protocols (MPP) to remain in effect, the Court has prevented dangerous chaos at the southern border, avoided a significant escalation in public health threats, and mitigated damage to foreign relations,“ Grisham said in a statement. “MPP ... has been crucial to our success working with Mexico to control rampant illegal migration, smuggling, and cartel-driven human trafficking, all of which present wide-ranging risks to both our countries and to the migrants themselves.“

Critics of the policy contend that requiring most asylum applicants to return to Mexico puts them at risk, particularly from violence related to rampant drug-trafficking in many parts of the country.

“The Court of Appeals unequivocally declared this policy to be illegal. The Supreme Court should as well,“ said the American Civil Liberties Union‘s Judy Rabinovitz, one of the lawyers for the challengers. “Asylum seekers face grave danger and irreversible harm every day this depraved policy remains in effect.“

The Trump administration began implementing the policy in January 2019, requiring most non-Mexican asylum seekers who are intercepted at the southern border or present themselves at checkpoints to return to Mexico to await asylum hearings in the U.S.

About three months later, however, a District Court judge in San Francisco ruled the policy was probably illegal and granted a preliminary injunction blocking it.

Last year, a panel of 9th Circuit judges agreed to a Justice Department request to put that ruling on hold, so implementation of the policy continued unabated, with 60,000 people being sent back to Mexico to wait over the past year or so.

But last month, another panel of the same appeals court said it was clear that the policy was illegal and should be blocked.

That ruling would have applied across the southern border, but after the Trump administration asked for a stay, the 9th Circuit agreed to limit its decision to states in its jurisdiction — Arizona and California — while the litigation proceeded further.

The Justice Department said the ruling last month triggered a surge toward the border that could eventually include many of the 25,000 asylum applicants awaiting hearings. Trump administration lawyers also told the Supreme Court the move to limit the ruling to two states would simply result in many asylum seekers shifting to the west to approach the Arizona and California border checkpoints.

In addition to warning about the security implications of a large rush on the border, the Trump administration told the 9th Circuit and the justices that the rulings against the program were undermining the U.S.-Mexico relationship and complicating diplomatic relations with the country.

