Arizona argued that the law cooperatively assists the U.S. with immigration enforcement, but the court mostly agreed with the Obama administration that key parts of the law encroach on the federal government's exclusive right to make policy on the matter.
Justice Antonin Scalia emphatically came to Arizona's defense in his dissenting opinion.
"Today's opinion, apÂproving virtually all of the Ninth Circuit's injunction against enforcement of the four challenged provisions of Arizona's law, deprives States of what most would consider the defining characteristic of sovereignty: the power to exclude from the sovereign's territory people who have no right to be there," Scalia wrote. "Neither the Constitution itself nor even any law passed by Congress supports this result."
Overall, the decision is an election-year victory for President Obama, who led the challenge.
The ruling is a blow for Republican Gov. Jan Brewer and serves as a warning shot to other state legislatures supportive of similar measures. It could help energize conservatives, who are strongly supportive of the law. Brewer's initial reaction focused on the fact that the papers-checking provision wasn't struck down.
"After more than two years of legal challenges, the heart of SB 1070 can now be implemented in accordance with the U.S. Constitution," she said, warning supporters of the law that legal challenges "will continue."
On the other hand, the decision could help Democrats galvanize Hispanics, who would be disproportionately targeted by the law and broadly oppose it.
"With three out of the four provisions being struck down, the ruling shows that the Obama administration was right to challenge this law, which was not just ill-advised but also unconstitutional," said Senate Majority Leader Harry Reid (D-NV). "Looking ahead to the immigration debate, it is disturbing that Mitt Romney called the unconstitutional Arizona law a 'model' for immigration reform."
Read the Supreme Court's decision on S.B. 1070: