Roberts joined in every word of Kennedy's majority opinion that struck down three contentious provisions of the law, but allowed one to go forward — the requirement that police check the immigration status of people they stop for other reasons.
Roberts could have sided with the conservatives who favored allowing other provisions to take effect, but that would have left the court divided 4-4, a distasteful outcome because it amounts to a waste of the justices' time.
What's more, Roberts might have reasoned that Kagan probably would side with the other liberal justices and Kennedy in some future immigration case over similar state laws that she could take part in. Such a case down the road probably would produce an outcome similar to what the court achieved last week in the Arizona case. But if Roberts stuck with the conservative justices, that scenario would have deprived him of a chance to try to influence the majority opinion.
Yet as the court followed its usual practice of issuing a torrent of opinions just days before the justices scatter for the summer, Roberts did not exactly abandon his frequent allies on the right.
He wrote a sharp dissent from Kagan's majority opinion barring mandatory sentences of life without possibility of parole for people younger than 18. The chief justice and the other conservatives rebuffed pleas from supporters of campaign spending limits and from liberal justices to take a fresh look at the 2-year-old Citizens United decision, perhaps the most politically unpopular decision of Roberts' tenure.
The court struck down Montana's limits on corporate campaign spending because the majority, including Roberts, said the state law conflicted with the Citizens United decision.
Montana Attorney General Steve Bullock called the court just "another political body." Liberal groups bemoaned the loss of a chance to carve out state exceptions to the Citizens United ruling that unleashed a tidal wave of big money contributions during this election year.
Roberts has called for consensus in judicial decisions since he arrived at the court in September 2005. He has advocated judicial modesty, though his critics insist he himself cast that aspiration aside in Citizens United and other high-profile cases involving abortion rights, race and gun rights.
Other predictions of furious discord among the ideologically divided justices in big cases were sometimes proved wrong. The justices avoided a major confrontation over the landmark civil-rights-era Voting Rights Act in an 8-1 decision that resolved the case without reaching disputed constitutional issues.
When the court heard a dispute over congressional redistricting maps in Texas that essentially pitted Republicans against Latinos and African-Americans who argued for greater representation, the court seemed headed for its typical conservative-liberal split. Instead, 11 days after hearing arguments, the court returned a consensus, unsigned opinion that gave both sides some of what they wanted.
Paul Clement, who argued the health care, immigration and redistricting cases, said the Texas case was a remarkable accomplishment for Roberts. "It was not foreordained as a unanimous decision. One has to assume it was largely due to the leadership of the chief justice," Clement said.
So what does the future hold?
The court already has agreed to hear a challenge to the University of Texas' affirmative action program. A new case involving the Voting Rights Act and challenges to restrictions on gay marriage are close behind.
Roberts has voiced serious reservations about racial preferences in government programs. In 2007, he declared in ruling against public school system programs to promote integration that "the way to stop discrimination on the basis of race is to stop discriminating on the basis of race."
Ben Wittes, a legal scholar at the Brookings Institution think tank in Washington, cautioned against grand re-evaluations of Roberts because of the health care case.
"Roberts is, to be sure, no shrinking violet about ideologically divided opinions when, in his view, the law compels them, but he apparently has a more flexible view than do his conservative colleagues concerning the difference for constitutional purposes between a penalty and a tax," Wittes said. "In other words, don't be too surprised if Roberts next terms looks like a conservative again. He actually did not stray very far from where the other four conservatives ended up in this case — just over a consequential line."
Roberts repeated his desire to have the court adhere to judicial modesty Thursday at the start of his health care opinion.
"We do not consider whether the act embodies sound policies. That judgment is entrusted to the nation's elected leaders. We ask only whether Congress has the power under the Constitution to enact the challenged provisions," he wrote.
Thursday's dissenters accused Roberts of "vast judicial overreaching." Several years ago, Justice Stephen Breyer said of Roberts and the other conservatives at the end of an especially contentious term with an unusually high number of 5-4 decisions that "never have so few done so much" in so little time.
Roberts has been mocked for comparing a justice to a baseball umpire calling balls and strikes.
But umpires often take it as a compliment when both teams are griping about their rulings.
Supreme Court interactive: http://hosted.ap.org/interactives/2012/supreme-court
- Photo gallery: Christmas around the world
- Officer kills armed 18-year-old near Ferguson
- Our six heroes of 2014
- NORAD 'tracks' Santa's journey across globe
- Ex-President George H.W. Bush to stay...
- On Christmas, Obama marks end of Afghan combat
- Final goodbye: A roll call of some who died...
- Capacity crowds pack screenings of 'The...
- NYPD: Cop ambush killer told passers-by... 15
- Going it alone, Obama rocked the boat... 13
- Officer kills armed 18-year-old near... 10
- Pope in blistering critique of Vatican... 9
- Key developments in the case of slain... 9
- Killings of 2 New York officers trigger... 6
- Senate, CIA agree torture program was... 6
- Sony announces limited release for 'The... 5