There is little doubt about Kennedy's mark on history. Quite simply, he remade the face of marriage in America. More than any other justice, he was responsible for the advancement of LGBT rights. He wrote four of the court's opinions on the subject over nearly two decades, and ultimately declared marriage between two people of the same sex a fundamental right protected by the Constitution.
"No union is more profound than marriage, for it embodies the highest ideals of love, fidelity, devotion, sacrifice, and family," Kennedy said in his opinion. "In forming a marital union two people become something greater than what they once were."
On a court that has become increasingly conservative, Kennedy's role has been pivotal. In 5-4 decisions, his vote has usually determined the outcome on some of the hottest legal and social issues of the day — not just gay rights, but abortion, campaign finance, gun rights, affirmative action, the war on terror and the death penalty.
President Trump has pledged to nominate a replacement who will almost certainly vote differently on many of these issues, putting some landmark decisions in jeopardy — from Roe v. Wade to the legalization of same-sex marriage.
That will please and reward conservative groups that have supported him. At the same time, it will galvanize opposition among Democrats, and potentially even some moderate Republicans.
So this time, the president will make his choice knowing that he only needs to get a majority of the Republican-controlled Senate to approve his nominee. And Senate Majority Leader Mitch McConnell, R-Ky., has proved adept at keeping his troops in line on judicial nominations.
Still, the pressure will be intense, and the stakes high.
If anyone has doubts about how different the court would likely be when a centrist conservative like Kennedy is replaced by someone more hard-line, there is the example of Justice Sandra Day O'Connor's retirement in 2006. President George W. Bush chose Samuel Alito to replace O'Connor, and the effect has been profound; O'Connor has even complained privately that Alito is systematically dismantling her legacy.
Perhaps the best example of that dismantling is in the area of campaign finance. O'Connor, onetime GOP leader of the Arizona Senate, voted to uphold landmark campaign finance legislation in 2003 — only to see the court reverse that decision seven years later, after Alito replaced her.
While O'Connor was on the court, a majority of justices continued to support legislation that regulated campaign fundraising. But after her retirement and the Alito appointment, Kennedy wrote the court's 5-4 decision in Citizens United, the 2010 case that remade the way campaigns at every level are conducted.
The decision unleashed an ever-growing flood of cash into political campaigns. It reversed a century-old understanding that had sought to prevent corruption by barring corporations, and later labor unions, from spending their general treasury funds on candidate elections.
For Kennedy, reversing that legal understanding from the early 1900s was the realization of a long-held view of free speech.
"Political speech is indispensable to decision-making in a democracy," Kennedy wrote in the Citizens United decision, "and this is no less true because the speech comes from a corporation rather than an individual."
In general, there is probably no greater example of Kennedy's approach to the law, and how it will almost certainly differ from Trump's nominee, than the question of how to interpret the Constitution: whether the Founding Fathers intended their creation as a static document, bound by the literal meaning of its words, or whether those words represent concepts of liberty to be interpreted over time.
In a 2003 decision that struck down a Texas law criminalizing private homosexual conduct, Kennedy said that the Founders understood that they were writing a document for the ages.
"They knew time can blind us to certain truths," he wrote in Lawrence v. Texas, "and later generations can see that laws, once thought necessary and proper, in fact serve only to oppress."
The men and women on Trump's list of potential replacements disagree with that view, for the most part. They side with the late Justice Antonin Scalia and his successor, Justice Neil Gorsuch, as well as other conservatives on the Supreme Court today, who believe the nation is bound by the original intent of the Founders.
During his presidential campaign, in a bid for the support of social conservatives, Trump issued two lists totaling 21 names and pledged to pick his first Supreme Court nominee from those lists. He did just that in naming Justice Gorsuch to fill the open seat once occupied by Scalia.
People involved in that selection process say that the president intends to expand that list a bit now. He might add Judge Brett Kavanaugh from the U.S. Court of Appeals for the D.C. Circuit, and possibly former Solicitor General Paul Clement, a highly regarded Supreme Court advocate who served in the George W. Bush administration. Clement's main drawback in the president's mind, though, is said to be that he has no judicial track record to examine.
Back in consideration are some of the names on the previous list: Judge Thomas Hardiman, of the 3rd U.S. Circuit Court of Appeals, which covers Pennsylvania, Delaware, and New Jersey, and Judge Raymond Kethledge, who serves on the 6th Circuit, which covers a large part of the Midwest. Hardiman was the runner-up last time and is known as a staunch gun rights advocate. All four are in their early 50s.
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