Since the retirement of Justice Sandra Day O'Connor in 2006, Kennedy has solely occupied the Supreme Court's center, regularly pivoting between the bench's evenly split conservative- and liberal-leaning factions to cast the decisive swing vote on key cases.
And as the Supreme Court enters a busy stretch of its 2013-14 season, Kennedy is expected to deliver key votes on more split decisions, making him the most closely watched -- and arguably most influential -- justice on the high court.
"Nobody decides to be a swing vote. You become a swing vote because the rest of the court is polarized," said Jamie Raskin, a constitutional law professor at American University. "And there are certain issues where Justice Kennedy is the one justice who does not have a perfectly consistent and coherent philosophy."
At the time, the Reagan administration believed that Kennedy, who had served almost 13 years on the U.S. Court of Appeals for the Ninth Circuit, would be a dependable -- though not necessarily lockstep -- conservative voice on the court.
But in his 26 years on the Supreme Court, Kennedy has shown an ideological flexibility that routinely brings praise and scorn from both sides of the political spectrum.
Conservatives have characterized him as an unprincipled turncoat for his votes supporting gay rights, and liberals have labeled him a shill for corporate America. Yet both sides have lauded him for landmark decisions to their liking.
"Kennedy is less [ideologically] rigid" than other Supreme Court jurists, said Suzanna Sherry, a constitutional law professor at Vanderbilt University.
"He is basically a pragmatist in the sense that he doesn't have some grand theory on how to decide constitutional cases," she said. "He ... draws on whatever sources are relevant for a particular case."
Moderates like Kennedy stand little chance of reaching the bench today, legal experts say, as presidents of both parties are increasingly eager to shape the Supreme Court in their political Image.
"He's a throwback to the Republican Party as it existed during the time of President Reagan and before," said Douglas Parker, a retired attorney who worked for the Nixon and Ford administrations and has written extensively about Kennedy. "There is, I think, more demand for ideological conformity" in the GOP today.
A high-profile example of Kennedy's judicial dichotomy occurred in June, when he ruled in the majority in two landmark 5-4 cases.
In one, he voted to strike down a key provision in the 1965 Voting Rights Act, a move that freed states of certain federal oversight of elections. His decision was applauded by conservatives, who viewed the provision as an encroachment on states' rights.
A day later, he delighted liberals when he wrote the majority opinion to throw out the federal Defense of Marriage Act. The ruling prohibits the federal government from denying benefits to same-sex couples who are legally married in states that recognize same-sex marriage.
"He's harder to pigeonhole than some of the justices on this current court," said Amy Howe, editor of the SCOTUSblog, a popular website that covers the Supreme Court. "He's definitely in the mix on every 5-4 case."
But Howe and other legal experts say Kennedy is actually quite predictable, as his rulings typically follow patterns and personal precedent.
The justice has been a dependable ally of conservatives on issues ranging from economic matters to corporate regulation to voting rights to gun control, but siding with liberals on civil liberties cases, especially those involving gay rights.
"What a lot of academics and politicians say about [Kennedy] is he's completely unpredictable and that he has no principles, that he just decides the cases the way he wants them to come out and that he's basically not following the law. I think that's wrong," Sherry said.
"There is a deep legal analytical logic to a lot of his opinions -- not all of them, but certainly a large percentage of them."
He wrote the majority opinion in the landmark Citizens United case in 2010 that struck down most limits on corporate spending in elections on the grounds that they violated First Amendment guarantees of free speech.
And he particularly pleased conservatives with his majority decision in Bush v. Gore, a 5-4 decision that resolved the disputed 2000 presidential election that led to President George W. Bush's victory over Al Gore.
Meanwhile, Kennedy's rejection of DOMA last year wasn't the first time he helped advance gay rights. He wrote the court's opinion in the landmark 1996 case Romer v. Evans that struck down a provision in the Colorado constitution denying gays the right to bring local discrimination claims. Seven years later, he ruled in the majority on Lawrence v. Texas, a case that invalidated sodomy laws, making same-sex sexual activity officially legal nationwide.
|"He is basically a pragmatist in the sense that he doesn't have some grand theory on how to decide constitutional cases. He ... draws on whatever sources are relevant for a particular case."|
"You've had this pretty traditional Roman Catholic in his 50s and 70s become a hero of the gay rights movement," Howe said. "This is not something he sort of begrudgingly went along with. ... You can tell that he feels quite strongly about [these rulings] and that he's writing them with an eye toward the ages."
Kennedy's judicial philosophy is nuanced even within some individual issues. He has supported the constitutional right to an abortion, for example, though he has ruled to uphold several restrictions on that right, including laws to prohibit partial-birth abortions.
"Justice Kennedy still floats a little bit of partisanship with the Republican Party, but he is not so neatly aligned with the partisan divide on the court," said Raskin, who is a Democratic Maryland state senator.
Kennedy, married with three children, was born and raised in Sacramento, Calif. After graduating from Stanford University and Harvard Law School, he began his legal career in private practice in San Francisco in 1961 before taking over his father's Sacramento law firm when he died in 1963.
Despite his moderately conservative upbringing, Kennedy has said he considers changes in public opinion -- particularly those involving gay rights -- when making his rulings.
"It's simply stunning to me to see the changes in attitudes” regarding sexual orientation, Kennedy said last year at the University of Pennsylvania. “It's something I didn't think about or know about as a kid. ... But the nature of injustice is, you can't see it in your own time."
This year, Kennedy is poised to provide more key votes on several marquee cases, including one on whether President Obama violated the Constitution when he made recess appointments to the National Labor Relations Board. The justices' decision could cast a legal cloud over hundreds of rulings by the board, which resolves complaints of unfair labor practices and conducts elections for labor union representation.
Social conservatives generally have lashed out at Kennedy more fiercely than liberals have, as they portray his opinions on gay rights, abortion and others as a betrayal. After the DOMA ruling, Family Research Council President Tony Perkins characterized Kennedy as a unprincipled judicial activist who should be impeached.
"I think he has breached the line of separation between the court -- the judicial branch -- and Congress and has taken on the policy role-making," Perkins said. "It's outrageous and I think that man should be removed from the bench."
Despite such criticism, legal experts say history will judge Kennedy kindly because of his ideological flexibility -- not in spite of it.
"I think [history] will see him as an island of stability in the court over some difficult and divisive times," Parker said.
Parker added he doubts Reagan would be surprised by Kennedy's career on the Supreme Court.
"I don't think Reagan would be upset with [nominating] Kennedy," he said. "He might not consider him his finest appointment, but I don't think he'd lose any sleep over it."