WASHINGTON — There is a reason Republican senators are so adamant in their refusal to let President Barack Obama appoint a successor to Justice Antonin Scalia, a towering figure in conservative jurisprudence. An Obama appointment would be the most consequential ideological shift on the court since 1991, creating a liberal majority that would almost certainly reshape American law and American life.
"At-risk precedents run from campaign finance to commerce, from race to religion, and they include some signature Scalia projects, such as the Second Amendment," said Lee Epstein, a law professor and political scientist at Washington University in St. Louis.
"Some would go quickly, like Citizens United, and some would go slower," she said. "But they'll go."
Every time the party in the White House changes, there is a potential impact on the court as the new president tries to mold it when openings develop. But Obama has a relatively rare opportunity to make a third appointment at a crucial moment. "The court is now divided on many issues," the president said Tuesday. "This would be a deciding vote."
The last nomination with the ideological stakes so high was in 1991, when Thurgood Marshall retired, by some measures the most liberal justice in modern history. He was replaced by Justice Clarence Thomas, the most conservative by those same measures, after a brutal confirmation process.
"The importance of the change in the court's jurisprudence that is directly attributable to the choice of Clarence Thomas to fill the vacancy created by Thurgood's retirement cannot be overstated," Justice John Paul Stevens, who retired in 2010, wrote in a memoir published in 2011.
Appointing a replacement for Scalia could be just as consequential. Abortion rights would become more secure, and gun rights less so. Business interests would meet with less success, and consumers and workers with more. Judicial hostility to government programs aimed at helping disadvantaged minorities would wane. First Amendment arguments in cases on campaign finance, public unions and commercial speech would meet a more skeptical reception.
Nothing would change overnight, but a lot would change with time. In the decade since the replacement in 2006 of Justice Sandra Day O'Connor, a moderate, by Justice Samuel A. Alito Jr., a conservative, for instance, the Supreme Court's jurisprudence on race, abortion, campaign finance and religion has shifted because of the swap.
A new Obama appointee would bolster the current court's four-member liberal bloc — Justices Ruth Bader Ginsburg and Stephen G. Breyer, who were appointed by President Bill Clinton, and Justices Sonia Sotomayor and Elena Kagan, appointed by Obama.
By political science measures, those four justices are ideologically very close. They often vote together in closely divided cases, and they have lately had some success in attracting a fifth vote for their positions, often from Justice Anthony M. Kennedy, who would almost certainly lose his position at the court's ideological center should Obama succeed in appointing the next justice.
Until Scalia's death, the four Democratic appointees were outnumbered by five Republican appointees, all of them more conservative. A fifth liberal vote could be profoundly consequential, said Vikram Amar, the dean of the University of Illinois College of Law.
"Adding another justice who has instincts and outlooks similar to those of Justices Ginsburg and Sotomayor could call into question a number of contentious 5-to-4 precedents," he said, ticking off the decisions that might be overruled.
Amar listed six, starting with Citizens United v. Federal Election Commission, which amplified the role of money in politics, and District of Columbia v. Heller, which recognized a Second Amendment right to keep guns.
The others, he said, were Shelby County v. Holder, which struck down a key provision of the Voting Rights Act; Glossip v. Gross, which upheld a lethal-injection protocol used in executions; Burwell v. Hobby Lobby, which allowed some corporations a religious exemption from a requirement to provide insurance coverage for contraception; and Town of Greece v. Galloway, which upheld prayer at town hall meetings.
That is a reasonably comprehensive list of the greatest hits of the court led since 2005 by Chief Justice John G. Roberts Jr. The major omissions are decisions in areas in which the more liberal justices have already prevailed. Those victories included rulings on same-sex marriage and the constitutionality of a key part of the Affordable Care Act.
Indeed, Amar said, "the judicial debate over the fundamental permissibility of Obamacare would likely draw to an end" with the appointment of a fifth liberal.
Epstein identified 85 decisions in the Roberts court years in which Scalia was part of a five-justice majority and Ginsburg and Breyer (the two liberals who served throughout those years) were in dissent.
Her list included several decisions that pleased business groups: Walmart Stores v. Dukes, which limited the availability of class actions; Ashcroft v. Iqbal, which made it easier for trial judges to dismiss cases at the earliest stages of a litigation; and AT&T Mobility v. Concepcion, which upheld a fine-print contract requiring consumers to arbitrate their cases one by one.
The list also included cases on abortion, school integration and government surveillance. And it featured at least five cases striking down campaign finance regulations.
The Democratic presidential candidates, Hillary Clinton and Sen. Bernie Sanders, have pledged to work to overturn the Citizens United decision. But that may not be enough to produce a fundamental change, said Tom Ginsburg, a law professor at the University of Chicago.
"Citizens United is on every liberal's list of opinions that ought to go," he said. "But the problems in campaign finance run deeper than that, perhaps back to Buckley v. Valeo," the foundational decision in 1976 on financing campaigns.
"Comprehensive reform may require more than just overturning a single case," Tom Ginsburg said.
There are practical and prudential reasons a new liberal majority might not embark on such a broad project. Appropriate cases presenting the relevant issues must reach the court, for starters, which can sometimes be a matter of happenstance.
Then there is the doctrine of stare decisis, which generally counsels against overruling settled law.
At his confirmation hearings in 2005, Roberts explained that "adherence to precedent promotes evenhandedness, promotes fairness, promotes stability and predictability."
But in Citizens United, he voted with the five-justice majority to overrule two important precedents. Stare decisis, he wrote in a concurrence, is not an "inexorable command."
It can be a malleable concept, Alito joked last September. "It is a Latin phrase," he said. "It means 'to leave things decided when it suits our purposes.'?"
He added, apparently more seriously, that it is "not difficult for a judge to make the stare decisis inquiry come out however the judge wants it to come out."
Eric J. Segall, a law professor at Georgia State, said that change might nonetheless come slowly and subtly.
"All the justices to varying degrees are sensitive to the charge that they are simply politicians in robes," he said. "Overturning cases because the balance of power has shifted does not happen right away or sit well with them."
Instead of overruling precedents outright, he said, a liberal majority might hollow some of them out, notably in the area of gun rights. "The five would narrow Heller to the point of irrelevancy," he said, speaking of the law that said Americans had a constitutional right to keep handguns at home.
Tom Ginsburg said a court dominated by liberals could reshape Second Amendment rights without overruling the Heller decision and its 2010 successor, McDonald v. Chicago.
"If we got a fifth liberal on the court, the pendulum would swing pretty quickly on gun control," he said. "I expect that we'd see a major shift in the kind of gun control laws that get approved by the court. Look for enhanced registration requirements as the first step."
Scalia wrote the majority opinion in Heller, and he grew frustrated by what he saw as lower courts' lawless refusal to follow it by sustaining all manner of gun control laws.
A new Obama appointee would make successful Second Amendment challenges to gun control laws even less likely, a point Sen. Ted Cruz has been making on the campaign trail.
"We are one justice away," Cruz said at a recent Republican presidential debate, "from a Supreme Court that will reverse the Heller decision, one of Justice Scalia's seminal decisions, that upheld the Second Amendment right to keep and to bear arms."