While Scalia seemed the most persistent supporter of Arizona's efforts, his views were at least shadowed by the questions posed by Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr.
"It seems to me the federal government just doesn't want to know who's here illegally and who's not," Roberts told the Obama administration's chief attorney at one point.
The state's law requires that Arizona law enforcement personnel who have detained people for other reasons check their residency status if the officers have a "reasonable suspicion" that they are illegal immigrants. When someone is arrested, residency status must be confirmed before the person is released.
Arizona's Support Our Law Enforcement and Safe Neighborhoods Act also makes it a state crime to be in the United States without authorization, as well as for an illegal immigrant to work or seek work without authorization. The law's supporters characterize it as a way to enforce immigration policies through "attrition," as illegal immigrants are targeted and perhaps removed from the United States.
The Obama administration went to court to block the provisions, arguing that the federal government preempts the state.
"The Constitution vests exclusive authority over immigration matters to Congress," Solicitor General David Verrilli said, adding that "these decisions have to be made at the national level."
But even Justice Sonia Sotomayor, an Obama administration appointee, sounded skeptical of at least some of the administration's arguments, telling Verrilli at one point that one of his answers "terribly confused" her, and at another advising him that "you can see it's not selling well."
Sotomayor, joined by Justice Stephen G. Breyer, did sound concerned as they pressed Arizona's attorney, Paul Clement, on how long individuals might be detained while their residency status was being checked. Clement said the average check took no more than 12 minutes, while Verrilli countered that the total waiting time was closer to an hour.
With Arizona Gov. Jan Brewer and other officials in the audience, and with the justices seemingly caught up in the give-and-take, the court expanded the typical hour-long oral argument to about 80 minutes. Justice Clarence Thomas, following his usual practice, was the only justice not to ask questions.
South Carolina, Idaho, Florida, and 13 other states have allied with Arizona, arguing for the power to impose certain immigration measures if they choose. House Minority Leader Nancy Pelosi (D., Calif.) and more than five dozen other House Democrats filed their own brief attacking the law, as did labor unions and several dozen cities and counties. On both sides, dozens of amicus briefs press different points.
The court's decision probably will come in June, about the same time it's expected to rule on the administration's health-care law. While a decision to uphold the strict Arizona law would be a legal defeat for the Obama administration, some scholars predict that it could help President Obama by boosting election turnout among the nation's 21 million voting-age Hispanics.
Congressional Democrats this week underscored their intentions to make hay if the court upholds Arizona's law, as Sen. Charles E. Schumer (D., N.Y.) staged a hearing Tuesday and promised to introduce a bill rebutting the court. The measure would have no realistic chance of passing this year through a divided Congress, but it would sharpen a debate that Democratic strategists such as Schumer think can help them among Hispanic voters.
Justice Elena Kagan is not taking part as her former colleagues in the Solicitor General's Office try to block Arizona's law. If the eight remaining justices tie 4-4, the decision by the U.S. Court of Appeals for the Ninth Circuit to strike down the Arizona provisions will be upheld, though without setting national precedent.