David Cole is national legal director of the ACLU and a professor at Georgetown University Law Center.

Not since Clarence Thomas filled the seat vacated by Thurgood Marshall has a Supreme Court appointment been as consequential as Ruth Bader Ginsburg’s replacement is likely to be. If Amy Coney Barrett is confirmed, a die-hard conservative will once again replace a civil rights hero, and the resulting shift will be tectonic.

Every Supreme Court nomination is important. The justices serve for life and decide many of the nation’s most hotly contested disputes, from abortion to gun rights to racial justice. But this will be the first appointment since Ginsburg herself in 1993 took the seat of Byron R. White — a Democratic appointee who tended to vote with the conservatives — that a confirmation will shift a seat on the court from one side of the ideological spectrum to the other.

With the exception of Thomas, all of the current justices replaced justices who roughly shared their ideological commitments. There were differences in nuance between the appointees and their predecessors, to be sure. Brett M. Kavanaugh is a little more conservative than Anthony M. Kennedy was; Sonia Sotomayor is a little more liberal than David Souter was. But in essence each appointment was one-for-one.

If, as seems all but inevitable at this point, a committed conservative replaces a committed liberal, the new justice will fundamentally alter the court’s ideological balance, giving it six conservatives and three liberals.

To appreciate what that could mean in practice, consider how many cases in the past decade or so have been decided 5 to 4, with Ginsburg providing a fifth vote. In June Medical v. Russo, the court struck down state restrictions on the right to abortion. In Obergefell v. Hodges and United States v. Windsor, the court recognized a right to marry on behalf of same-sex couples. The court has twice preserved affirmative action from constitutional challenge by the same one-vote margin.

Also by a single vote, the court struck down the death penalty and mandatory life-without-parole sentences for juvenile offenders; and declared that, for adult offenders, the death penalty may be imposed only for homicide and not rape. It ruled that detainees at Guantanamo Bay Naval Base in Cuba have a constitutional right to judicial review of their detention. Two years ago, again by the narrowest margin, the court protected Americans’ rights to the privacy of their cellphone location data. Last year, the court invalidated President Trump’s partisan attempt to add a citizenship question to the census, a move designed to aid Republicans and harm Democrats. This past term, the court stopped Trump’s effort to lift “dreamers’” protections from deportation.

Granted, many important cases were decided 5 to 4 in the conservative direction — including decisions invalidating campaign finance laws, striking down the most important provision of the Voting Rights Act, recognizing an individual right to bear arms, protecting businesses from lawsuits, and of course, stopping the Florida recount to hand the contested election of 2000 to George W. Bush over Al Gore.

With a court composed of six conservatives and three liberals, it would be extraordinarily difficult for liberals to win, and conservatives to lose, virtually any ideologically charged dispute. And with a court so unbalanced, and likely out of step with the public, attacks on its legitimacy will increase.

Most immediately, the Affordable Care Act is in jeopardy. The ACA, which has extended critical health insurance to millions of Americans and bars insurers from discriminating against those with preexisting conditions, has survived two constitutional challenges: the first by a 5-to-4 margin, the second 6 to 3. In California v. Texas, the Trump administration is arguing that the ACA should be thrown out — including its protection of those with preexisting conditions. The case rests on a highly implausible legal argument, but given how partisan the battles over Obamacare have been, it has a chance of succeeding before a court of six conservatives.

What can be done? Some are already urging that Democrats pack the court if they win the presidential election and control of the Senate. That approach failed the last time it was tried, even though it was proposed by the highly popular Franklin D. Roosevelt, at a time when the Supreme Court was the most unpopular it has ever been. Such a move would likely spark a political arms race that would make the court just another highly politicized institution, to the detriment of us all. Other reforms, such as giving justices 18-year staggered terms so that every president gets to appoint two justices, are more balanced, and may deserve consideration.

But the most important response will come at the ballot box. Studies show that throughout its history, the court has rarely diverged substantially from where public opinion is on fundamental constitutional issues. So if the country moves to the left, or at least away from the right, in response to the norm-violating, truth-defying presidency of Donald Trump, the Supreme Court would be unlikely to obstruct widely supported social reforms. The answer, for now, is to vote like your rights depend on it.

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