For the past decade on the Supreme Court, Justice Anthony Kennedy wielded more influence over the lives and destinies of millions of Americans than any single person who wasn’t president. He may have saved his most profound impact on the nation for last.

Kennedy, who announced his retirement from the Supreme Court on Wednesday, was the deciding voice in dozens of major cases during his tenure. He leaves behind four reliably liberal colleagues and four reliably conservative colleagues. That pivotal ninth seat will now be filled by a more doctrinaire conservative jurist chosen by President Donald Trump, who gets to name a second appointment to the nation’s highest court.

“For a member of the legal profession it is the highest of honors to serve on this Court,” Kennedy wrote in a letter notifying Trump of his decision. “Please permit me by this letter to express my profound gratitude for having had the privilege to seek in each case how best to know, interpret, and defend the Constitution and the laws that must always conform to its mandates and promises.”

The effect of Kennedy’s departure will be immediate and long-lasting. For most of his tenure, the 81-year-old justice carved out a moderate conservative voice on the nation’s legal disputes. Whoever replaces him is guaranteed to be a far less flexible figure. Instead, the court’s next justice will almost certainly help transform the court’s conservative wing into a sledgehammer for the American right.

Kennedy followed in past justices’ footsteps by retiring at the end of the Supreme Court’s annual term, giving the president and the Senate until the first week of October to choose and confirm a replacement. Trump told White House reporters on Wednesday that he would start the process to select a nominee “as quickly as possible.” Senate Majority Leader Mitch McConnell said the Republican-led chamber would also act speedily to fill the vacancy. “We will vote to confirm Justice Kennedy’s successor this fall,” he said in a floor speech.

Democrats are effectively powerless to prevent Trump from confirming another selection to the high court on their own. Republican senators already killed the judicial filibuster when Democrats invoked it in a futile effort to block Neil Gorsuch’s confirmation last year. With a 51-seat majority, Republicans would need to lose vote from two of their own senators for a nomination to fail. Considerable outside pressure will likely fall on Maine Senator Susan Collins and Alaska Senator Lisa Murkowski, two pro-choice Republicans, to reject a nominee who might threaten abortion rights.

Kennedy’s retirement is a crushing defeat for the Democratic Party and for liberal policymaking. In 2016, Antonin Scalia’s death and that year’s presidential election gave Democrats an opportunity to place a fifth justice nominated by their party on the Supreme Court for the first time since 1969. Trump’s victory instead allowed Republicans to first solidify the court’s existing conservative majority, and now push it even further to the right. A liberal majority on the Supreme Court is now out of reach for at least a generation, maybe longer. Instead, every major policy program that the American left hopes to enact—Medicare for All, universal college tuition, and more—will have to face a gauntlet of conservative jurists in order to survive.

Such was Anthony Kennedy’s influence over the course of American life. He tacked rightward more often than not: Kennedy frequently sided with the court’s conservative wing on cases involving federalism, executive power, and the rights of criminal defendants. Many of Kennedy’s major 5-4 votes helped entrench the Republican Party’s political power. His majority opinion in Citizens United v. FEC set the stage for a tidal wave of dark money to wash over American politics. He sided with Chief Justice John Roberts in Shelby County v. Holder, which gutted part of the Voting Rights Act of 1965 and prompted GOP-led states to pass dozens of restrictive voting measures. Earlier this month, he helped turn back an effort to establish constitutional limits on partisan gerrymandering.

But he also often sided with the court’s liberals on major cases involving social issues. Those rulings preserved a delicate status quo between the American left and right that will now likely collapse in his absence. Kennedy’s middle-of-the-road stance on abortion rights led him to uphold some restrictions on the practice while also thwarting efforts to reverse Roe v. Wade entirely. In 2016, he voted with the court’s liberals to defeat a long-running legal campaign that curbed affirmative action in higher education. With him gone, conservatives will likely mount new efforts to push the court rightward on both issues. Republican-led states will more aggressively restrict abortion rights in the hope of triggering a Supreme Court showdown. Four states already have laws to ban abortion automatically if Roe is overturned.

On at least one front of the cultural wars, Kennedy served as a field marshal for the liberal side. He wrote the majority opinion in every major Supreme Court case on LGBT rights starting with Romer v. Evans in 1996, which struck down an anti-gay constitutional amendment in Colorado. Those cases led to the legalization of same-sex marriage nationwide in the 2015 case Obergefell v. Hodges, where Kennedy laid out a vision of equal dignity under the Constitution for gay and lesbian Americans. His recent vote in the Masterpiece Cakeshop case marked a step back from that legacy, though not a total repudiation of it.

Kennedy’s replacement on the high court is unlikely to share his predecessor’s flexibility. During the 2016 campaign, Trump distributed a list of 25 conservative legal figures and pledged he would only choose Supreme Court nominees from among them. The move aimed to shore up support among Republicans who feared Trump’s ideological flexibility would be reflected in his judicial nominees, imperiling a decades-long effort to reshape the nation’s courts in their image.

Neil Gorsuch, Trump’s first nominee to the Supreme Court, laid any lingering fears among Republicans to rest. Since joining the high court last year, Gorsuch has been a folksy and consistent voice in lock-step with the rest of the court’s conservatives. For many Republicans, the pick helped them justify brushing aside Trump’s numerous scandals and attacks on the rule of law. Conservative legal figures who gathered at this year’s Federalist Society convention even received red stress balls emblazoned with the newest justice’s image and the refrain “But Gorsuch!”

Trump’s other potential nominees appear likely to follow in his first choice’s footsteps. By stepping down from the court, Kennedy leaves behind a complex record on the court—one that largely maintained a tenuous balance of power between America’s increasingly polarized political factions. That balance now appears to be in jeopardy: upsetting it could be Anthony Kennedy’s most lasting legacy.