Washington - The Supreme Court on Monday rejected much of Arizona's tough new immigration law but allowed one key provision to stand, saying federal law did not pre-empt the state's instruction to its police to check the immigration status of people they detain.
Several other important provisions of the law conflicted with federal laws, the court found, rejecting provisions that made it a state crime for immigrants not to register with the federal government or to seek or hold jobs without proper documents, and to make warrantless arrests of some people suspected of being deportable.
The Obama administration had urged the court to strike down the whole law, including its provision requiring state law enforcement officials to determine the immigration status of people they stop and suspect are not in the United States legally. That provision also requires that the immigration status of people who are arrested be determined before they are released.
Except on the one provision -- to be sure, the one that had seized the most public attention -- the Supreme Court's decision affirmed the finding of a San Francisco appeals court that had blocked the Arizona statute from taking effect for the time being. It sent the case back to the appeals court.
Monday's decision, written by Justice Anthony M. Kennedy, was 5 to 3; Justice Elena Kagan was recused because of her previous role as solicitor general. Justices Samuel A. Alito Jr., Antonin Scalia and Clarence Thomas dissented.
But on the question of allowing the status checks, the court was unanimous.
Even after the Supreme Court's ruling that one key provision was not automatically pre-empted, immigration groups will be able to challenge it based on an argument that the court was not considering: that the law discriminates on the basis of race and ethnic background.
The Arizona law undertook what it called a policy of "attrition through enforcement." Conservatives have pressed for tough measures to stem illegal immigration, including ones patterned after the Arizona law, in Alabama, Georgia, Indiana, South Carolina and Utah. President Obama has criticized the Arizona law, calling it a threat to ''basic notions of fairness.''
Arizona's attempt to make it a crime for immigrants to work or to try to find work is an example of how the statute conflicted with the will of Congress. Federal law penalizes employers who hire illegal workers but does not punish employees for working.
"The court correctly held that federal immigration law trumps most of Arizona's controversial immigration law," Stephen W. Yale-Loehr, who teaches immigration law at Cornell University and is co-author of a treatise on the subject, said. "But by upholding Arizona's 'check your papers' provision, at least for now, the court has given other states a green light to try to enact similar immigration laws."
"Some will be anti-immigrant, like Arizona's," Professor Yale-Loehr said. "But other new state laws may be pro-immigrant, as states realize the importance of immigrants in their communities. The decision increases pressure on Congress to enact comprehensive immigration law to prevent a crazy patchwork of conflicting immigration laws around the country."
The decision came as the court began what is expected to be the final week of its session, and involved a case that was a rematch between the main lawyers in the most vigorously disputed and eagerly awaited verdict, President Obama's 2010 health care law.
Paul D. Clement, who argued for the 26 states challenging the health care law, represented Arizona. Solicitor General Donald B. Verrilli Jr. represented the federal government, as he did in defending the health care law.
In oral arguments earlier this year, the two lawyers presented sharply contrasting accounts of what the Arizona law meant to achieve. Mr. Clement said the state was making an effort to address a crisis by passing a law that complemented federal immigration policy. ''Arizona borrowed the federal standards as its own,'' he said, adding that the state was simply being more assertive in enforcing federal law than the federal government.
Mr. Verrilli countered that Arizona's approach was in conflict with the federal efforts. ''The Constitution vests exclusive authority over immigration matters with the national government,'' he said.
After the law was passed in 2010, the Obama administration quickly sued to block it, saying it could not be reconciled with federal laws and policies. In legal terms, the case is about whether federal law ''pre-empts,'' or displaces, the challenged state law.
Last year, a three-judge panel of the United States Court of Appeals for the Ninth Circuit, in San Francisco, blocked four provisions of the law on those grounds.
Also last year, in Chamber of Commerce v. Whiting, the Supreme Court held that a different Arizona law, this one imposing harsh penalties on businesses that hire illegal workers, was not pre-empted by federal law. The vote was 5 to 3, and it split along ideological lines.