WASHINGTON -- A clash over immigration law will go before the U.S. Supreme Court this week, pitting the state of Arizona against President Barack Obama in a case with election-year political ramifications for him and Republican rival Mitt Romney.

In its second-biggest case this term, the court -- fresh from hearing the Obama healthcare overhaul case -- will consider on Wednesday whether a tough Arizona immigration crackdown strayed too far into the federal government's powers.

A pro-Arizona decision would be a legal and political setback for Obama, who has criticized the state's law and vowed to push for immigration legislation if re-elected on Nov. 6.

A decision against Arizona would deal a blow to Romney, who has said the government should drop its challenge to the law.

Americans generally support immigration laws like Arizona's and are ambivalent about the federal and state roles at the core of the case, a new Reuters/Ipsos opinion poll found.

About 70 percent of those surveyed favored state laws that let police check a person's immigration status and make it a crime for an illegal immigrant to work in the United States; about 30 percent opposed such measures.

On the question of who has responsibility for immigration laws, the core of the Supreme Court case, 59 percent said immigration was a national issue and laws relating to it should only be made by the federal government; 55 percent said individual states had the right to make such laws, too.

The oral arguments on this essential point set the stage for a rematch of the attorneys in last month's healthcare battle.

Paul Clement, a solicitor general during Republican George W. Bush's presidency, will represent Arizona.

Donald Verrilli, a former White House lawyer and solicitor general under Obama, will represent the federal government after what some deemed a lackluster performance in March.

PARTISAN DIVIDE

The case "pits a politically conservative state against a Democratic administration . .. just six months before the presidential-year elections," said Steven Schwinn, a John Marshall Law School professor.

Like the healthcare case, the immigration case splits along party lines. Some Republican-led states and Romney supported Arizona's effort to push out illegal immigrants, while some Democratic-led states backed the federal government and Obama.

Legal and political experts said the Supreme Court's rulings, expected by June on immigration and healthcare -- two hot-button issues -- will weigh heavily on the elections.

The online Reuters/Ipsos poll of 960 Americans, conducted April 9-12, found respondents almost evenly split on whether Obama or Romney has a better immigration approach. The margin of error was plus or minus 3.7 percentage points for all respondents.

The fast-growing Hispanic population, now equal to 16 percent of all Americans, will be a key force in the election. In the past, Hispanics backed Obama and have been skeptical of Romney, who has acknowledged that he needs their support.

Regardless of how the court rules, if Romney continues to back the Arizona law and others like it, "he will find it difficult to win Latino supporters," said Karthick Ramakrishnan, an immigration policy expert and associate professor of political science at the University of California, Riverside.

FEDERAL VS STATE

At issue in the case is whether federal immigration law pre-empted and thus barred the Arizona law's four key provisions.

The Arizona law requires police to check the immigration status of anyone detained and suspected of being in the country illegally. Other parts of the law require immigrants to carry their papers at all times; ban illegal immigrants from soliciting for work in public places; and allow police to arrest immigrants without a warrant if an officer believes they have committed a crime that would make them deportable.

A federal judge and a U.S. appeals court earlier ruled for the Obama administration and blocked all four parts of the Arizona law from taking effect.

Clement will argue that the Arizona law was designed to cooperate with federal immigration efforts and that it did not conflict with federal policy or law.

"This is another federalism case. This is not all about immigration. It's really about the relationship between the federal government and the state government. It's the norm that you have state officials enforcing federal law," he said in an interview with Reuters.

Clement said the burden was on the government to show why immigration law specifically prevented states from the usual participation in enforcement of federal policy.

There are an estimated 11.5 million illegal immigrants in the United States, a number that has remained steady over the last several years.

SENSITIVE IMMIGRATION JUDGMENTS

For his part, Verrilli has a policy of not commenting on arguments he will make before the Supreme Court.

In written briefs filed with the court, Verrilli said that Congress gave the federal government alone the authority to make sensitive immigration judgments, balancing national security, law enforcement, foreign policy and humanitarian factors, along with the rights of law-abiding citizens and immigrants.

"Arizona seeks to impose its own judgment on those sensitive subjects," Verrilli said. "For each state and each locality to set its own immigration policy in that fashion would wholly subvert Congress's goal: a single national approach."

Five other states -- Alabama, Georgia, Indiana, South Carolina and Utah -- have followed Arizona's lead and adopted similar laws, parts of which could be affected by the Supreme Court's ruling. In some of those states, legal immigrants have faced run-ins with local law enforcement.

Legal experts said the Supreme Court could uphold all four provisions; rule that all four were pre-empted; or issue a mixed ruling, allowing some provisions to stand, but not others.

"If the court upholds the Arizona statute, then that will really be a significant change in that area of the law," said Douglas Hallward-Driemeier, who leads the appellate and Supreme Court practice at the law firm Ropes & Gray.

"There are a number of precedents going back many decades that immigration regulation is a matter of foreign affairs reserved to the federal government," he said.

The Supreme Court last year upheld a different Arizona law that penalizes businesses for hiring illegal immigrants. But that case involved a different pre-emption issue.

KAGAN RECUSED

The case this week will be heard by eight of the nine Supreme Court members. Justice Elena Kagan has recused herself, apparently because she worked on the case in her previous job as Obama's solicitor general. In the event the court is evenly divided on the case, the appeals court ruling for the federal government would be affirmed.

The partisan battle was underscored by the outside briefs filed by the states and lawmakers. Two U.S. senators and 57 members of the House of Representatives, all Republicans, backed Arizona, while 68 Democratic members of Congress supported the position of the president.

Seventeen foreign countries, including Mexico and others in Central and South America, backed the U.S. government. Clement said the appeals court was wrong to allow foreign criticism of the law to influence its ruling.

No matter the Supreme Court ruling, it may not be the final word. Depending on the decision, Congress could rewrite federal law to allow more state regulation or clearly pre-empt it.

If the court rules for Arizona, Cecilia Wang of the American Civil Liberties Union vowed to press ahead with claims saying that the law violated constitutional rights and was based on illegal racial profiling. "Lawful U.S. citizens will be caught up in the dragnet," she predicted.



If the court rules against Arizona, supporters of tougher state laws could try to craft new measures.



The Supreme Court case is Arizona v. United States, No. 11-182.



