President Barack Obama should nominate a Supreme Court justice who is as liberal as Chief Justice John G. Roberts Jr. and Justice Samuel Alito are conservative. It cannot be that Republican presidents can select staunch conservatives with impunity, yet Democratic presidents can choose only moderates.

No one was surprised by Justice John Paul Stevens's announcement that he would step down at the end of this term. He will be 90 years old on April 20, and his resignation has been predicted for months. Nor was there surprise when Republican senators Orrin Hatch of Utah and Mitch McConnell of Kentucky, within hours of Stevens's announcement, cautioned that Obama should not appoint an "activist."

It’s hard to believe that these two senators can say as much with a straight face, in light of the performances of Roberts and Alito on the Court. Both justices have been aggressive activists in their own willingness to overrule precedents, to strike down laws, and to rule broadly. They were key votes in five-person majorities to strike down campaign finance laws and gun control ordinances, advancing the conservative ideological agenda in what, by any definition, were highly activist decisions.

At the very least, Obama should nominate someone at least as concerned about individual liberties and equality as Stevens is. Although he was left of center, Stevens was not as liberal as justices William J. Brennan or Thurgood Marshall. For instance, in recent years, Stevens ruled (over dissents by other liberal justices) that execution by lethal injunction is not cruel and unusual punishment and that a state may require photo identification as a prerequisite for voting.

What were some of the hallmarks of Stevens's constitutional jurisprudence? He is a strong advocate for the need for checks and balances, such as in his rulings that Guantánamo detainees have access to federal court via a writ of habeas corpus and that the military commissions created by President George W. Bush violated the law. Stevens believes that there should be a wall separating church and state — a position strongly opposed by conservatives. He dissented from Rehnquist-era decisions limiting Congress's power to enact civil rights laws and provide for their enforcement against state governments.

Stevens also has been a consistent vote to enforce the Constitution's protections for criminal defendants, often in his dissenting opinions. In 2002, I argued a case before the court on behalf of Leandro Andrade, who had been sentenced to 50 years to life in prison for stealing $153 worth of videotapes from Kmart stores in San Bernardino, Calif. Andrade received the sentence under California's three strikes law, even though he had never committed a violent or serious crime. I lost in a 5-4 ruling, with Stevens writing for the dissent and explaining that no one in American history had ever received a life sentence for shoplifting until California's draconian law.