The court is evaluating the 2010 law on only the question of whether Arizona's attempt to fix its border problems is trumped by federal law. That means that opponents could still ask the courts to block enforcement of the law on other legal grounds.
For example, the high court isn't considering the possibility that racial profiling may arise from the law - because the Obama administration's lawsuit didn't challenge it on those grounds. The administration focused instead on whether federal law supersedes the state law, an issue known as "pre-emption."
"All the court is going to decide is the pre-emption issue," said Linton Joaquin, general counsel for the National Immigration Law Center, an advocacy group for low-income immigrants that's part of a coalition of opponents that filed a separate challenge. "But we think this law basically requires racial profiling by mandating that officers detain and investigate people that they have reasonable suspicions of being unauthorized."
The case was argued before the high court in April, and a ruling is expected by the end of June. Based partly on skeptical questions posed by justices during the hearing, legal experts expect that the court likely will uphold Arizona's requirement that police check the immigration status of people they stop for other reasons; that provision was put on hold by a judge in July 2010 and hasn't yet been enforced. Less controversial parts of the law were allowed to take effect.
A decision in favor of Arizona could clear the way for other states to enforce immigration-check requirements and create an opening for states to take a larger role in immigration enforcement after mostly staying out of it for decades and letting the federal government handle it alone.
Five other states - Alabama, Georgia, Indiana, South Carolina and Utah - have enacted similar laws.
If Arizona wins at the Supreme Court, opponents of the law say they likely would go back to lower courts to seek injunctions on other grounds before any provisions that win approval from the Supreme Court take effect. They also may ask the courts to block enforcement of the law's most controversial parts by arguing that the law requires police to extend the length of time of traffic stops beyond the permitted time.
"We are preparing for the next step in case of a bad decision," said Andre Segura, an attorney for the American Civil Liberties Union, which also is fighting the law in court.
Segura said it's unlikely that a Supreme Court decision that upholds the Arizona law would go into effect immediately. Instead, the case would probably be given back to lower courts to decide when it takes effect, though it's unclear how long it would take for the courts to decide when police can start enforcing the provision.
It's also unclear when similar laws in other states would take effect if the court rules in Arizona's favor. Those states will likely have to take up the issue in their respective courts to see how their laws - which have different provisions on the questioning of people's immigration status - square up with the Supreme Court's ruling, Segura said.
Lawyers who are fighting the law aren't the only opponents preparing themselves for a ruling that upholds the law.
Immigrant rights advocates plan to launch a public relations campaign in hopes of quelling fears about the law and hold public meetings across the state to explain the law. They also are planning protests and a bus tour across the country to protest Arizona-style immigration laws. And a hotline run by a civil rights group will take questions about the law and document reports of abuses by police.
Meanwhile, Maricopa County Sheriff Joe Arpaio - who, more than any other police boss in Arizona, has pushed the bounds of local immigration enforcement - said he has no plans to expand his immigration efforts if Arizona wins its appeal. "I really don't see any big change with me," Arpaio said, adding that his officers already ask people for their immigration status when they have a good reason to do so.
The U.S. Justice Department has accused Arpaio's office in a lawsuit of racially profiling Latinos in his trademark immigration patrols. The sheriff vigorously disputes the allegation.
Along the Arizona-Mexico border, Santa Cruz County Sheriff Tony Estrada doubted his deputies will need to undergo special training to enforce the law's questioning requirement. He said his deputies, who now call the U.S. Border Patrol to pose such questions, would instead inquire about people's status and, if needed, call federal agents for assistance.
Estrada, who has 37 deputies to patrol a county that shares 50 miles (80 kilometers) of border with Mexico, said he will have to balance enforcement of the law with his limited manpower.