After serving on the Supreme Court for more than 30 years, Justice Anthony Kennedy has announced his retirement -- leaving an open seat on the nation's highest court for President Trump to fill.

Kennedy, 81, said his retirement will be effective July 31. Last year, his former law clerk Neil Gorsuch took over the Supreme Court seat once occupied by the late Justice Antonin Scalia.

Read on for a look at Kennedy's time on the nation's highest court and what's next for the vacant seat.

Kennedy has been on the Supreme Court for more than 30 years

Kennedy first took his seat on the Supreme Court on February 18, 1988. He was nominated by former President Ronald Reagan, a Republican.

He’s been a Supreme Court justice longer than any current member. He also has joined the majority opinion of more split cases than any other justices; according to a 2015 Rolling Stone profile, Kennedy joined the majority 84 percent of the time when a case was split 5-4.

Kennedy wasn't always considered to be a swing vote on the court

By the end of his tenure, Kennedy was considered to be a swing vote on the court -- although that wasn't always the case. When he was first seated, Kennedy was considered to be part of the court's more conservative block.

In 2006, the Washington Post called the nation's highest court "a Supreme Court of One" after Kennedy decided to side with the court's more left-leaning faction regarding former President George W. Bush's military tribunals for Guantanamo Bay detainees.

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In April 2018, The New York Times' editorial board publicly asked Kennedy to avoid retiring, saying, "Your record is more conservative than liberal, but there's no question that you are less of an ideologue than anyone President Trump would pick."

The White House praised Kennedy's "landmark opinions in every significant area of constitutional law"

In a statement regarding his retirement, the White House praised Kennedy for having "authored landmark opinions in every significant area of constitutional law, most notably on equal protections under the law, the separation of powers and the First Amendment's guarantees of freedom of speech and religion."

In 1992, Kennedy joined Justices David Souter and Sandra Day O'Connor in co-writing the Supreme Court's opinion in Planned Parenthood of Southeastern Pennsylvania v. Casey. That case upheld women's right to have an abortion but also gave states more authority in how to regulate it.

In keeping with his penchant for believing in individual rights, Kennedy agreed with the court's decision to legalize same-sex marriage in 2015. And in 1996, Kennedy wrote for the majority as the court struck down a Colorado amendment to its state constitution that prohibited local governments from enacting laws that would protect LGBT citizens.

"The right to marry is a fundamental right inherent in the liberty of the person, and under the Due Process and Equal Protection Clauses of the Fourteenth Amendment couples of the same-sex may not be deprived of that right and that liberty," Kennedy wrote in the majority opinion for Obergefell v. Hodges in 2015, which legalized gay marriage. "The Court now holds that same-sex couples may exercise the fundamental right to marry. No longer may this liberty be denied to them."

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In 2016, Kennedy upheld the University of Texas' affirmative action program in Fisher v. University of Texas.

And in Roper v. Simmons, a 2005 case, Kennedy wrote the majority for the Court in banning capital punishment for minors.

"It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty," he said then.

Kennedy is a graduate of Harvard Law School

Kennedy graduated from Harvard Law School in 1961, after having attended Stanford University and the London School of Economics.

In 1962, Kennedy was admitted to the California Bar and practiced law first in San Francisco before moving back to his hometown of Sacramento. Kennedy also served as an adjunct professor at the McGeorge School of Law for nearly 20 years, beginning in 1965, according to the Supreme Court Historical Society.

Former President Gerald Ford appointed Kennedy to the U.S. Court of Appeals for the Ninth Circuit in 1976.

Kennedy grew up as a well-behaved kid. He was so well-behaved, in fact, that his father reportedly once offered him $100 if he could get himself arrested. Kennedy supposedly never collected on that bet.

Kennedy also served in the Army National Guard.

Expect this vacancy to become a major point in this year's midterms

A Supreme Court vacancy will likely become a key issue in a midterm congressional election year, when control of the Senate is at stake.

That body will consider Trump's latest high court nominee, requiring only a simple majority for confirmation. GOP leaders changed the rules when Neil Gorsuch was being considered, to get rid of the 60-vote procedural filibuster threshold.

But Republicans only have a slight majority in the Senate -- 51-49, which includes ailing Sen. John McCain of Arizona.

Trump has promised to begin the nomination process immediately, pledging to pick from a list of 25 names he started during his presidential campaign and updated last year.

Fox News' Shannon Bream, Bill Mears and The Associated Press contributed to this report.