WASHINGTON The Supreme Court on Monday delivered a split decision on Arizona’s tough 2010 immigration law, upholding its most controversial provision but blocking the implementation of others.

The court unanimously sustained the law’s centerpiece, the one critics have called the “show me your papers” provision. It requires state law enforcement officials to determine the immigration status of anyone they stop or arrest if there is reason to suspect that the individual might be an illegal immigrant.

The justices parted ways on three other provisions. Justice Anthony Kennedy, writing for five members of the court, said the federal government’s broad powers in setting immigration policy meant that other parts of the state law could not be enforced.

“The national government has significant power to regulate immigration,” Kennedy wrote. “With power comes responsibility, and the sound exercise of national power over immigration depends on the nation’s meeting its responsibility to base its laws on a political will informed by searching, thoughtful, rational civic discourse.”

Kennedy added: “Arizona may have understandable frustrations with the problems caused by illegal immigration while that process continues, but the state may not pursue policies that undermine federal law.”

Obama ‘pleased’

The decision was a partial victory for the Obama administration, which had sued to block several parts of the law.

In a statement released later Monday, President Barack Obama said he was “pleased” with the court’s decision to strike down some aspects of the law, but he voiced his concern about the remaining provision.

“I agree with the court that individuals cannot be detained solely to verify their immigration status. No American should ever live under a cloud of suspicion just because of what they look like,” Obama said. “Going forward, we must ensure that Arizona law enforcement officials do not enforce this law in a manner that undermines the civil rights of Americans.”

Presumptive Republican presidential nominee Mitt Romney said he disagreed with the court’s decision to strike down parts of the law. He told about 200 donors in Arizona that, in the absence of federal action, states should have broader power to craft their own immigration measures.

“Given the failure of the immigration policy in this country, I would have preferred to see the Supreme Court give more latitude to states, not less,” Romney told contributors at the Scottsdale Plaza Resort. “States now, under this decision, have less authority, less latitude to enforce immigration laws.”

Federal trumps state

Monday’s ruling was a partial rebuke for state officials who argued that they were entitled to supplement federal efforts to address illegal immigration.

The administration’s legal arguments were based on asserted conflicts between the state law and federal immigration laws and policies. The question for the justices, then, was whether federal immigration law trumped – pre-empted, in the legal jargon – the state efforts.

Last year, a three-judge panel of the U.S. Court of Appeals for the 9th Circuit, in San Francisco, blocked four provisions of the law on those grounds.

The administration did not challenge the law based on equal protection principles. At the Supreme Court argument in the case in April, Solicitor General Donald Verrilli Jr., representing the federal government, acknowledged that his case was not based on racial or ethnic profiling.

Monday’s decision in Arizona v. United States, No. 11-182, did not foreclose further lawsuits based on that argument.

In sustaining one provision and blocking others, the decision amounted to a road map for permissible state efforts in this area.

Several other states have enacted tough measures to stem illegal immigration, including ones patterned after the Arizona law in Alabama, Georgia, Indiana, South Carolina and Utah.

Lower courts have stayed the implementation of parts of those laws, and they will now revisit those decisions to bring them in line with the principles announced Monday.

Dissenting opinions

Three justice dissented. Justices Antonin Scalia and Clarence Thomas said they would have sustained all three of the blocked provisions. Justice Samuel Alito would have sustained two of them.

Scalia read a lengthy dissent from the bench.

“After this case was argued and while it was under consideration,” he said, “the secretary of Homeland Security announced a program exempting from immigration enforcement some 1.4 million illegal immigrants.”

This was a reference to the decision by the Obama administration this month to let younger immigrants – the administration estimates the number as approximately 800,000 – who came to the United States as children avoid deportation and receive working papers as long as they meet certain conditions.

“The president has said that the new program is ‘the right thing to do’ in light of Congress’ failure to pass the administration’s proposed revision of the immigration laws,” Scalia went on.

“Perhaps it is, though Arizona may not think so. But to say, as the court does, that Arizona contradicts federal law by enforcing applications of federal immigration law that the president declines to enforce boggles the mind.”

Justice Elena Kagan disqualified herself from the case, Arizona v. United States, No. 11-182, presumably because she had worked on it as Obama’s solicitor general. Bloomberg News contributed.