The U.S. Supreme Court in a 5-3 decision ruled Monday that a key part of the Arizona immigration law that requires police to check a person’s immigration status if they have reasonable suspicion that the person is in the country illegally is constitutional.

The provision is one of four sections of the law the High Court considered. The court struck down the part of the law that made it a state crime to be in Arizona without immigration papers because it infringed on federal jurisdiction.

"Today’s decision by the U.S. Supreme Court is a victory for the rule of law...After more than two years of legal challenges, the heart of SB 1070 can now be implemented in accordance with the U.S. Constitution," said Arizona Gov. Jan Brewer.

A ban forbidding employment of all undocumented workers in the state was also ruled unconstitutional, along with a provision that would allow police to arrest, without a warrant, anyone believed to have committed a crime that would lead to deportation, even if the crime was committed in another state.

Although the court upheld what many consider to be the most significant portion of the law, the opinion suggested that the law could be reviewed again depending on how law enforcement determines the qualifications for “reasonable suspicion” that a person is in the country illegally.

“There is a basic uncertainty about what the law means and how it will be enforced,” Justice Anthony Kennedy wrote in the majority decision. “At this stage, without the benefit of a definitive interpretation from the state courts, it would be inappropriate to assume §2(B) will be constructed in a way that creates a conflict with federal law.”

Chief Justice John Roberts joined Kennedy in the majority decision, along with Justices Stephen Breyer, Sonia Sotomayor, and Ruth Bader Ginsburg. Justice Elena Kagan recused herself from the case. Justices Antonin Scalia, Clarence Thomas, and Samuel Alito all wrote separate dissenting opinions, with Scalia and Thomas arguing that the entire law should have been upheld and Alito arguing that he would have upheld three portions of the law and struck down one.

In his dissent, Scalia argued that the court infringed on Arizona’s sovereign powers by striking down any portion of the law. “If securing its territory in this fashion is not within the power of Arizona, we should cease referring to it as a sovereign state,” Scalia wrote.

The case has become a symbol of a larger national debate about immigration, which has emerged as a hot election year topic as President Barack Obama and rival Mitt Romney both attempt to appeal to the Hispanic voting bloc, which many analysts are saying will be the deciding factor in the 2012 presidential election.

While the ruling is in part a defeat for Obama, some scholars believe that the court’s unwillingness to strike down the entire law could have positive political implications for the president by increasing turnout in next fall’s election among the nation’s 21 million voting-age Hispanics.

“Fear,” Gary Segura, a Stanford University pollster and political science professor, said earlier this year, “is a remarkable mobilizer.”

The court's opinion for Arizona v. United States is below.

  Ariz v U S (PDF)
Ariz v U S (Text)