Washington - The federal government, not states, holds responsibility for enforcing immigration laws, the U.S. Supreme Court ruled, striking down three key parts of Arizona's crackdown on illegal immigrants and jeopardizing similar laws in five other states.

The justices did clear the way for Arizona to begin enforcing another controversial part of its law, which directs police to check the immigration status of people they suspect are in the country illegally when they make lawful stops for other reasons. But the justices approved that provision only after attaching conditions that sharply limited its reach.

The invalidated provisions would have allowed Arizona authorities to arrest illegal immigrants for looking for work or for not carrying registration documents. Justices also voided a provision authorizing police to arrest immigrants for offenses that may warrant deportation.

Arizona vs. United States marked the most important effort in decades to draw a line between federal and state authority over illegal immigrants. An estimated 11 million people are living and working in this country without legal authorization. What to do about them has become an intensely contested political issue, with Republican leaders in Arizona and elsewhere calling for more aggressive enforcement of immigration laws.

Arguing that the federal government was doing too little, Arizona officials passed the state's immigration law, SB 1070, two years ago. Backers argued that giving state authorities a legal basis to arrest and jail illegal immigrants would cause the immigrants to leave the state - even if federal agents did not want to deport them. Five other states - Alabama, Georgia, South Carolina, Indiana and Utah - passed similar measures. The Obama administration sued and won rulings in lower courts suspending the key provisions.

Monday's high court decision allowed advocates on both sides to claim partial victory. But the 5-3 majority opinion written by Justice Anthony Kennedy and joined by Chief Justice John G. Roberts Jr. and three of the court's liberals tilted heavily toward the federal government, leaving little role for the states. Overall, it was a significant victory for the Obama administration before a conservative court.

"Arizona may have understandable frustrations with the problems caused by illegal immigration," Kennedy wrote, "but the state may not pursue policies that undermine federal law."

Juvenile justice

In other action, the court concluded that it is cruel and unusual punishment to send a young murderer to "die in prison" if a judge has not weighed whether he deserved a shorter prison term because of his youth and the nature of his crime, the Supreme Court ruled Monday.

The 5-4 decision struck down laws in 28 states that mandate life terms for juvenile murderers with no hope for parole.

The ruling could lead to new sentencing or a parole hearings for more than 2,000 prisoners around the country who committed homicides when they were young and were given life terms under mandatory laws.

Campaign finance

The justices on Monday also affirmed that corporations have the right to engage in direct political activity, squelching a long-shot hope of campaign finance reform advocates of rolling back Citizens United, the 2010 decision that ushered in a new era of unbridled campaign spending.

In a 5-4 decision, the court summarily overturned a century-old Montana state law that banned corporate political expenditures, saying it conflicted with the First Amendment speech rights of corporations affirmed in the Citizens United case.

The ruling, while expected, served to underline the bleak landscape facing proponents of campaign finance regulations just a decade after the landmark McCain-Feingold Act restricted large, unregulated donations in federal elections.

Now, not only can corporations spend unlimited amounts of money on politics, but the 2012 campaign has been marked by the explosive growth of "super PACs"-independent political committees created by a lower-court decision that followed Citizens United and fueled by wealthy individuals and private companies. And with Monday's action, the Supreme Court made it clear that any state laws seeking to ban corporate political expenditures are unconstitutional.

Such spending is "threatening the health of our democracy," said White House spokesman Eric Schultz, adding that the court missed an opportunity to correct the "mistake" it made in Citizens United.