By James Vicini and Jonathan Stempel
WASHINGTON (Reuters) - The U.S. Supreme Court upheld a key part of Arizona's crackdown on illegal immigrants on Monday but struck down three other parts of the state law, delivering a mixed ruling for the Obama administration on federal power to enforce immigration statutes in the United States.
In an opinion by Justice Anthony Kennedy, the country's highest court by an 8-0 vote upheld the Arizona law's most controversial aspect, requiring police officers to check the immigration status of people they stop for any reason.
But in a split verdict, the justices also ruled that the three other provisions of the Western state's 2010 law that were challenged in court by the Obama administration went too far. The votes on those provisions were either 5-3 or 6-2, with the more conservative justices in dissent.
These three provisions required immigrants to carry immigration papers at all times, banned illegal immigrants from soliciting work in public places, and allowed police arrest of immigrants without warrants if officers believed they committed crimes that would make them deportable.
"Arizona may have understandable frustrations with the problems caused by illegal immigration ... but the state may not pursue policies that undermine federal law," Justice Kennedy wrote for the majority in a 25-page opinion.
The decision in part was an election-year setback for President Barack Obama, but was less of a defeat than had been envisioned after oral arguments before the justices in April. The majority opinion's sweeping rhetoric could cloud state efforts to try to curb illegal immigration.
Justice Kennedy stressed that the federal government has "broad, undoubted power" over immigration, pointing to how federal policy could affect trade, investment, tourism and diplomatic relations.
The majority that struck down three challenged parts of the Arizona law also included Chief Justice John Roberts, as well as the more liberal justices Ruth Bader Ginsburg, Stephen Breyer and Sonia Sotomayor, the nation's first Hispanic justice.
"I am pleased that the Supreme Court has struck down key provisions of Arizona's immigration law. What this decision makes unmistakably clear is that Congress must act on comprehensive immigration reform. A patchwork of state laws is not a solution to our broken immigration system - it's part of the problem," Obama said in a statement.
Obama said he remained concerned about the practical impact of the remaining provision of the Arizona law. "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, as the court's decision recognizes," Obama said.
In upholding the police checks, Kennedy said their mandatory nature did not interfere with the federal immigration scheme, and found unpersuasive the Obama administration argument that federal law preempted this part of the law at this stage.
He said it was improper to block that provision before state courts had an opportunity to review it, and without some showing that its enforcement conflicted with federal immigration law.
Kennedy also left open the possibility for constitutional or other challenges to the law once it goes into effect.
Opponents of the Arizona law also have sued on other grounds that it was unconstitutional and could lead to ethnic and racial profiling of the fast-growing Hispanic population, which represents 16 percent of all Americans.
The ruling went to the heart of a fierce national debate between Democrats and Republicans over the 11.5 million illegal immigrants the U.S. government estimates to be in the country.
The ruling upholding the police checks was a win for Arizona Republican Governor Jan Brewer, who championed the measure and called the decision a "victory for the rule of law."
Republican presidential candidate Mitt Romney, who challenges Obama in the November 6 U.S. election, had opposed the federal government's challenge to the law.
"Today's decision underscores the need for a president who will lead on this critical issue and work in a bipartisan fashion to pursue a national immigration strategy," Romney said in a statement.
SCALIA ANGRILY DISSENTS
Justice Antonin Scalia read an angry dissent from the bench, saying he would have upheld the entire Arizona law. It "boggles the mind" that the president might decline to enforce federal immigration law, Scalia said in apparent reference to Obama's June 15 executive order stopping deportation for certain young people in the United States illegally.
Obama has vowed to push for comprehensive immigration legislation if re-elected on November 6. Opinion polls show Hispanics, now equal to 16 percent of all Americans, overwhelmingly support Obama. Most illegal immigrants are Hispanics.
Arizona, on the southwest border with Mexico, two years ago became the first of half a dozen U.S. states to adopt laws to drive illegal immigrants out. About 360,000 of the country's illegal immigrants, or 3 percent, reside in Arizona. Most of the state's nearly 2 million Hispanics are in the country legally.
The immigration dispute was widely viewed as the second most important case in the Supreme Court's 2011-12 term, behind only the historic legal battle over Obama's healthcare overhaul law. A ruling in that case is expected on Thursday.
Roberts said from the bench that the court's last day of the term will be Thursday, and that all remaining opinions are expected to be issued that day. He did not specifically mention the healthcare case.
The court's ninth member, liberal Justice Elena Kagan, did not take part in the immigration ruling - believed to be because she worked on the case in her prior job as Obama's solicitor general.
Last year, the court upheld a different Arizona law that penalizes businesses for hiring illegal immigrants.
Obama on June 15 announced an important change in U.S. immigration policy. Hundreds of thousands of illegal immigrants who were brought into the United States as children could be able to avoid deportation and get work permits under the new policy.
The Supreme Court case is Arizona v. United States, No. 11-182.
(Reporting by James Vicini, Joan Biskupic and Jonathan Stempel; Editing by Howard Goller and Will Dunham)