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Phoenix • More than two years after it was signed into law, the most contentious part of Arizona's landmark immigration legislation is expected to go into effect after a federal court ruling issued Wednesday.

But the U.S. Supreme Court laid a legal minefield that Arizona now must navigate when the critical provision takes effect. The clause, one of the few significant ones that the high court left standing in a June ruling, requires all Arizona police officers to check the immigration status of people they stop while enforcing other laws and suspect are in the country illegally.

While preserving that requirement, however, the Supreme Court explicitly left the door open to arguments that the law leads to civil rights violations. Attorneys would need actual victims to make that case.

Civil rights activists are preparing to scour the state for such victims. Lydia Guzman, who runs Respect/Respeto, a Phoenix group that tracks racial profiling, said volunteers at the organization's call center have already been told to listen for new complaints when the requirement goes into effect.

"We're watching and we're looking for cases," she said.

Barring a successful, emergency challenge of Wednesday's ruling to an appeals court — an outcome that legal observers believe is unlikely — the requirement is expected to go into effect in the next several days. U.S. District Judge Susan Bolton rejected arguments by civil rights attorneys that she should prevent the requirement from kicking in, noting that the Supreme Court had specifically found that the provision should be allowed to become law.

Arizona police were formally trained on how to implement the law shortly after Gov. Jan Brewer signed it in 2010. The heads of some of the state's biggest law enforcement agencies — the Phoenix and Tucson police departments and the Pima County sheriff's office — were critical of it but ultimately said they would obey whatever parts the courts found to be constitutional.

"We enforce laws passed by our legislators," Sgt. Tommy Thompson, a Phoenix Police spokesman, said Wednesday night, noting the requirement still has not gone into effect.

Maricopa County Sheriff Joe Arpaio, who has been the most publicly aggressive in pursuing illegal immigrants, said in an interview Wednesday that his deputies already check the immigration status of people they encounter. Arpaio, a supporter of the law, said he expects no differences other than an increased number of lawsuits.

The law's author, former state Sen. Russell Pearce, said he does not expect sweeping changes in the way local police will conduct themselves once the requirement kicks in.

"I'm not asking for roundups, I'm not asking for anything but paying attention and doing your job," he said. "It's not that we want people in jail. We want compliance."

Peter Spiro, a Temple University law professor who has followed the two-year legal battle, said Arizona remains in a difficult legal spot.

"If the state's savvy at all, it's going to be very cautious" about how it implements the requirement, he said. "To the extent that it's not, it's going to be very vulnerable on this.

"Further litigation," Spiro said, "is imminent."

Since it overwhelmingly passed Arizona's Republican-controlled Legislature in 2010, the immigration law has been fiercely challenged in court.

Among its opponents was the Obama administration, which challenged the law based on the argument that federal immigration law trumped Arizona law. The challenge didn't confront racial profiling, and the administration failed to persuade the nation's highest court to strike down the questioning requirement.

To the supporters of Arizona's law, the questioning requirement was the most important part of the statute, whose stated purpose was to reduce the problems associated with illegal immigration through enforcement by the state.

Immigrant rights groups believe the requirement presents the most opportunities for civil rights abuses.

Shortly before the law was to take effect in July 2010, Bolton prevented police from enforcing the questioning requirement and other parts of the statute, ruling the Obama administration would likely succeed in showing federal law trumps the state law.

Brewer appealed the ruling, lost at the 9th U.S. Circuit Court of Appeals and took her case to the Supreme Court.

Less controversial sections of the law have been in effect since late July 2010 but have rarely been used.

Arizona's law was passed amid voter frustration with the state's role as the busiest illegal entry point into the country. Five states — Alabama, Georgia, Indiana, South Carolina and Utah — have adopted variations of Arizona's law.

Brewer's office said the law is expected to go into effect shortly.

"The courts have now consistently found that the plaintiffs have not met the high bar in arguing this law needs to be enjoined before it's allowed to take effect," gubernatorial spokesman Matthew Benson said. "Certainly, Gov. Brewer is pleased with this decision. She believes it's time SB1070 is implemented and so that we can see how effective this law is in practice."

Karen Tumlin, an attorney for the National Immigration Law Center, said her office was "considering our legal options" after Bolton's ruling.

"We were surprised and disappointed," said Dan Pochoda, legal director for the American Civil Liberties Union of Arizona.

Bolton did, however, grant a preliminary injunction against a statute making it illegal to harbor individuals suspected of being in the country illegally.

Guzman, the Arizona civil rights activist, said she expects police to tread cautiously as they implement the requirement.

"They know they're under the watchful eyes of activists like me, attorneys and even their own departments," she said.

Riccardi reported from Denver. Associated Press writers Walter Berry in Phoenix and Felicia Fonseca in Flagstaff contributed to this report.