The liberal Justice Kennedy is in the house -- at least for the moment.

Anthony Kennedy’s opinion for the U.S. Supreme Court embraced most of the Obama administration’s view that Arizona’s aggressive immigration laws are pre-empted by the federal government’s power to determine who is or is not allowed in the country. For good measure, Kennedy also provided the deciding fifth vote to rule that mandatory life without parole for murderers under 18 violates the Constitution.

In doing so, he might have given us a glimpse of some political gamesmanship behind closed doors at the court in the run-up to Thursday’s expected opinion on President Barack Obama’s signature health-care reform law.

The immigration decision squarely rejected three provisions of the Arizona law. One made it a state crime not to complete or carry the immigration papers already required by federal law. Another made it criminal for an illegal immigrant to apply for or hold a job -- a rule that goes beyond the federal norm that criminalizes the employer of an illegal worker, not the worker himself. The third provision gave the police the authority to arrest without a warrant anyone whom they reasonably suspected of having committed some act that would allow them to be removed from the country under federal immigration law. (Ordinarily, if federal authorities stop a removable alien, they can’t detain him, but simply tell him that he must show up for a hearing.) The court concluded that all of these provisions were beyond Arizona’s authority to enact.

Papers, Please

When it came to the most controversial part of the Arizona law -- the so-called “papers, please” provision -- the court did something rather clever. It said that the law might be read to mandate nothing more than a requirement for police to check immigration status during the ordinary duration of a legitimate stop based on reasonable suspicion of a crime. If that was all the law required, said the court, it would be constitutional.

On the surface, this holding upheld “papers, please.” In practice, though, the court took away some of the potentially frightening effects of the law, suggesting a roadmap for the state to follow in implementing it. The court said that detaining people only to determine their immigration status “would raise constitutional concerns,” implying that the length of the stop should not take longer than it ordinarily would to perform a criminal background check.

Whether this is a realistic directive remains to be seen. It is not clear what would constitute probable cause for an officer to think that a person who has been stopped is an illegal immigrant, if not something discriminatory like racial profiling. Thus the court left open the possibility of a future challenge by civil rights advocates should the law be implemented in a discriminatory fashion.

Justices Stephen Breyer, Ruth Bader Ginsburg and Sonia Sotomayor all joined Kennedy’s opinion. (Justice Elena Kagan recused herself because she worked on the case while serving as solicitor general.) Ordinarily, this would mean they were satisfied with Kennedy’s position on “papers, please.”

Yet this is not an ordinary Supreme Court term. The three liberals may have been willing to give Justice Kennedy more room to maneuver than usual because they wanted to keep him close for the health-care decision.

Changing Sides

Something of the kind may also have been in the mind of Chief Justice John Roberts, usually part of the conservative bloc. In a highly unusual move, Roberts joined Kennedy on the liberal side in the immigration case despite the dissenting votes of Justices Antonin Scalia, Clarence Thomas and Samuel Alito. We might read this as a sign that Roberts, too, was hoping to bring Kennedy to his side in the health-care decision. A few years ago, when Kennedy was a necessary fifth vote for extending his corpus rights to prisoners in Guantánamo, then-Justice Stevens joined Kennedy and the conservatives in an important case about treaty interpretation. Many observers interpreted his vote as a gesture toward Justice Kennedy.

There are other possible explanations for Roberts’ vote in the immigration case. Under the Supreme Court’s rules, with only eight justices sitting, a four-to-four tie would have meant affirming the decision of the United States Court of Appeals for the Ninth Circuit, which struck down all of the Arizona provisions including “papers, please.” By joining Kennedy’s opinion, Roberts at least assured that one of the law’s provisions would be formally upheld. There is a certain logic to this deduction, but it seems like a slender reed on which to hang Roberts’ uncharacteristic vote.

Roberts may also have been trying to shape his own image as a nonpartisan “umpire” -- a term he famously used about his future role in his confirmation hearings -- by voting in accordance with the Obama administration. If he intends to vote against the health-care bill, his supporters might be able to point to the Arizona case as proof that he is not partisan but objective.

In the end, however, this decision, like the health care decision ahead, came down to Justice Kennedy. Liberals may be excited about what this might mean for health care. But they should not get too excited. At the same time that it decided the immigration case and struck down mandatory life without parole for juveniles, the court issued a one-page per curiam opinion invalidating a Montana law that limited political contributions by corporations. The opinion, with a 5-4 ideological split, explained briefly that the Montana law must be struck down under the logic of the Citizens United case that extended First Amendment speech protection to corporate donations to candidates.

The Montana case had offered an opportunity for the court - - meaning Kennedy -- to revisit a ruling that gave rise to the super-PACS now dominating the presidential election. Had the court given the Montana case full consideration, it might have found some way to distance itself from the consequences of Citizens United. That it did not do so suggests Kennedy is perfectly comfortable with his role as the decisive conservative vote in that momentous decision.

The Arizona immigration case is a modest victory for the Obama administration’s decision to assert federal authority against the state law. It also suggests that Obama’s team was right not to press an antidiscrimination claim before the law had been implemented, a strategy that was widely criticized by the law’s detractors. But whether that sense of victory will last out the week is another question altogether.

(Noah Feldman, a law professor at Harvard University and the author of “Scorpions: The Battles and Triumphs of FDR’s Great Supreme Court Justices,” is a Bloomberg View columnist. The opinions expressed are his own.)

To contact the writer of this article: Noah Feldman in Cambridge, Massachusetts, at noah_feldman@harvard.edu.

To contact the editor responsible for this article: Tobin Harshaw at tharshaw@bloomberg.net.