WASHINGTON – Anthony Kennedy, the retiring Supreme Court justice, may have been appointed by a Republican president, but his opinions over the last three decades have been challenging to pigeonhole from a partisan perspective.

Kennedy has been a key – even swing vote – on high-profile issues including gay rights, abortion, affirmative action and campaign finance.

Here’s a sampling of some of his most notable opinions:

OBERGEFELL v. HODGES (2015)

In a landmark gay marriage decision, Kennedy wrote for a sharply divided court that same-sex couples have the right to marry anywhere in the United States.

The seminal ruling, which prompted cheers and dancing outside the courthouse, meant that individual states could not enforce bans on same-sex marriage. It also held that the Constitution’s promises of due process and equal protection guaranteed the right of gay couples to marry without legal barrier.

“Their hope is not to be condemned to live in loneliness, excluded from one of civilization’s oldest institutions,” Kennedy wrote of gay couples seeking to marry. “They ask for equal dignity in the eyes of the law. The Constitution grants them that right.”

UNITED STATES v. WINDSOR (2013)

The Obergefell case was hardly Kennedy’s only significant opinion on gay rights.

He wrote a 2003 opinion that invalidated Texas’ law against sodomy. Ten years later, Kennedy wrote a landmark decision that struck down a key section of a federal law that kept legally married same-sex couples from receiving tax, health and pension benefits.

The Windsor case concerned a section of the federal Defense of Marriage Act that said same-sex couples were barred from collecting federal marriage-related benefits.

In an opinion joined by the court’s four liberal justices, Kennedy said the primary effect of the law, known as DOMA, was to “identify a subset of state-sanctioned marriages and make them unequal.”

CITIZENS UNITED v. FEC (2010)

In arguably the most significant campaign finance ruling in the court’s history, Kennedy wrote the 5-4 opinion that opened door to greater political spending by independent groups.

The opinion removed caps from independent spending by corporations and labor unions.

Kennedy wrote for the deeply divided court that limits on spending by corporations and unions ran afoul of First Amendment protections.

“If the First Amendment has any force, it prohibits Congress from fining or jailing citizens, or associations of citizens, for simply engaging in political speech,” Kennedy wrote.

The opinion was publicly derided by President Barack Obama in a State of the Union address and was criticized by groups seeking to limit the influence of big money in politics.

BOUMEDIENE v. BUSH (2008)

The Boumediene case was one in a series of Supreme Court decisions that tested the legal rights of Guantanamo Bay detainees and the Bush administration’s authority to hold enemy combatants there.

In a 2008 opinion that covered centuries of European and American history, Kennedy held that detainees had the right to challenge their detention, and the basis for it, in court and before a judge.

“Within the Constitution’s separation-of-powers structure, few exercises of judicial power are as legitimate or as necessary as the responsibility to hear challenges to the authority of the Executive to imprison a person,” Kennedy wrote. “Some of these petitioners have been in custody for six years with no definitive judicial determination as to the legality of their detention. Their access to the writ is a necessity to determine the lawfulness of their status, even if, in the end, they do not obtain the relief they seek.”

ROPER v. SIMMONS (2005)

Kennedy wrote multiple notable opinions on capital punishment, including this 2005 ruling that said it was unconstitutional to execute defendants for crimes they committed when they were younger than 18.

The case concerned a gruesome Missouri murder and a 17-year-old defendant, Christopher Simmons, who was convicted along with a friend of kidnapping a woman from her home and throwing her body over a bridge and into a river.

Simmons was sentenced to death. But Kennedy held that it violated the Eighth Amendment’s prohibition against “cruel and unusual punishment” to execute people for crimes committed as juveniles.

“When a juvenile offender commits a heinous crime, the State can exact forfeiture of some of the most basic liberties, but the State cannot extinguish his life and his potential to attain a mature understanding of his own humanity,” Kennedy’s majority opinion said.