In the most divisive cases before the court in the term that just ended, it was Justice Anthony M. Kennedy who determined the outcome. In unpredictable fashion he sided some of the time with the court's conservative bloc and some of the time with its liberals.
His influence was displayed Thursday, when the court announced that it had struck down President Bush's specially created military tribunals for terrorism suspects.
As the decision was read, Roberts and Samuel A. Alito Jr., Bush's other new appointee, could only listen in agreement as Justices Antonin Scalia and Clarence Thomas read their dissents.
It was 86-year-old John Paul Stevens, the court's last World War II veteran, who read the 5-3 majority opinion. He solemnly declared that the president "is bound to comply with the rule of law" and may not ignore congressional mandates and long-standing U.S military rules.
He paused to note that Kennedy, seated next to him, had joined most of his opinion, creating a majority. Liberals hailed the result, and conservatives lamented it.
While the issue before the court was the balance of power in government, the drama showed how little the balance of power within the high court itself had changed. Even when Roberts and Alito are paired with fellow conservatives Scalia and Thomas, they need a fifth vote to prevail.
For the past decade, Justices Kennedy and Sandra Day O'Connor, both Ronald Reagan appointees, had supplied the votes that decided the court's major cases. They usually joined with the conservatives on issues of crime, the death penalty, civil rights and states' rights, but with the liberals on abortion, gay rights, school prayer and other social issues.
Now, with O'Connor in retirement, Kennedy stands alone at the center.
He voted with the conservatives more often than not but joined the liberals in several major rulings. In one closely watched environmental case, Kennedy wrote a separate, solo opinion that was decisive.
On the issue of military tribunals, Kennedy made clear he shared the liberals' concern with unchecked presidential power.
The Constitution created "a system where the single power of the executive is checked," he wrote. Even in a national emergency, he said, "the Constitution is best preserved by a reliance on standards tested over time and insulated from the pressure of the moment."
This does not mean that terrorism suspects cannot be tried in military tribunals, Kennedy said. But Congress should first debate the issue and pass a law.
Kennedy is hardly in the camp of the liberals. Just the day before, he spoke for a 5-4 majority that upheld the mid-decade redistricting plan engineered by former House Majority Leader Tom Delay for Texas' seats in the U.S. House of Representatives.
The four liberals, led by Stevens, say "partisan gerrymandering" is unconstitutional. The conservatives, led by Scalia, counter that politics inevitably plays a role in the drawing of electoral districts and that there is no fair way to decide how much politics is too much.
Kennedy came down in between. He says he finds partisan gerrymandering troubling, but the Texas plan was not so extreme as to be unconstitutional.
Before 2003, he pointed out, the Democrats had drawn electoral districts that gave them a slim majority of seats in Congress at a time when nearly 60 percent of Texans were voting Republican. Measured against that map, Delay's plan "can be seen as fairer," Kennedy said.
Developers and property-rights activists hoped a more conservative court, bolstered by Roberts and Alito, would sharply limit the Army Corps of Engineers' control over hundreds of millions of acres of wetlands. Environmentalists feared the same.
Scalia wrote an opinion to do that, and he was joined by Roberts, Thomas and Alito. He said federal environmental protection extended only to wetlands that were part of a continuous flowing stream. This would exclude much of wetlands in the middle part of the nation and nearly all those in West, where streams are dry for much of the year.
But in this case, Kennedy refused to go along. Instead, he wrote in a separate opinion that wetlands could be protected as long as environmentalists could show that filling them or draining them would affect downstream waters.
This year, Kennedy and O'Connor thwarted the Bush administration's move to void the nation's only "right to die" law. Oregon's voters had twice approved a measure that allows dying people to obtain a dose of lethal medication from their doctors.
Bush's first attorney general, John Ashcroft, reinterpreted the federal drug control law and said it empowered him to strip Oregon's doctors of their right to prescribe medication.
In January, Kennedy, speaking for the court, overturned Ashcroft's order. The administration's position, he said, would "delegate to a single executive officer the power to effect a radical shift of authority from the states to the federal government to define general standards of medical practice in every locality."
Kennedy, who turns 70 this month, joined the court after one of the most tumultuous confirmation battles in its history. In 1987, Reagan's first nominee, Robert Bork, was defeated in the Senate. His second, Douglas Ginsburg, withdrew after reports that he had smoked marijuana as a law professor.
Kennedy, a Sacramento native with a reputation as a straight arrow, was nominated next and won unanimous confirmation in the Senate.
At first, he looked to be a reliable conservative, voting regularly with Chief Justice William H. Rehnquist. But in 1992, he split with the conservatives and voted with O'Connor to uphold the right to abortion and to maintain the strict ban on school-sponsored prayers.
David G. Savage writes for the Los Angeles Times.