In a move signaling the erosion of the judicial cloister, Justice Scalia sparred publicly yesterday with the president of the American Civil Liberties Union at a Washington conference hosted by the famously litigious liberal group.

During the hour-long debate with the ACLU official, Nadine Strossen, the dean of the Supreme Court’s conservative wing warned that the notion of an “evolving” Constitution, which the group favors, was as likely to undermine individual liberties as it was to promote them.

“We don’t always get better and better. Sometimes we get worse and the purpose of the Bill of Rights was to prevent that,” Justice Scalia said.

Ms. Strossen suggested that the general trend of American history has been “toward more and more freedom and equality for more people.”

“Where is that one-way street, Nadine?” Justice Scalia shot back. “Where does it say the only way the Constitution, the Bill of Rights can evolve is in the direction of more freedom? It doesn’t say that anywhere. And you will find that it has evolved in both directions.”

Justice Scalia said a day will come when ACLU members will wish they had taken refuge in the doctrine of originalism, which he espouses. The doctrine calls for interpreting the Constitution based on the intent of the founders and of those who drafted constitutional amendments.

“Someday you’re going to get a very conservative Supreme Court and you’re going to regret what you’ve done,” Justice Scalia said.

Using a time-tested debate technique, Justice Scalia and Ms. Strossen each tried to push the other’s arguments to their logical extreme. This brought a concession by Ms. Strossen that bigamy laws were undermined by the Supreme Court’s 2003 decision finding constitutional protection against laws criminalizing gay sex.

Despite pressure from Ms. Strossen, Justice Scalia never quite conceded that the landmark Supreme Court decision outlawing racial segregation in public schools, Brown v. Board of Education, would not have happened if the doctrine of original intent was strictly followed.

However, the jurist said the fact that justices following a more flexible view of the Constitution identified the evil of segregation did nothing to undermine his warnings about leaving such decisions to the “aristocracy” of the court.

“A king would give you some good stuff that the untidy process of democracy will not produce,” Justice Scalia said. “That doesn’t prove it’s a good system just because now and then it has given you a good result.”

While Justice Scalia is usually at odds with the ACLU, he sometimes agrees with the group on free speech issues and the rights of criminal defendants. He has supported the right of American terrorist suspects to be charged in criminal courts and has urged that the constitutional right to confront witnesses be rigorously protected.

Ms. Strossen, a professor of law at New York University who has been head of the ACLU since 1991, repeatedly addressed Justice Scalia by his nickname, “Nino,” underscoring the unusually familiar tone of the debate. She said the two have known each other for more than a decade and agreed to use their first names in the discussion.

In the freewheeling exchange, Justice Scalia balked only once, when he refused to respond to Ms. Strossen’s question about whether the Constitution’s ban on racial discrimination would restrict the government’s ability to use race to focus anti-terrorism efforts.

“I better not talk about that,” he said. Principles of judicial ethics call for judges to refrain from discussing pending cases outside of court. Most judges also try to avoid opining on issues that are likely to come before them in the future.

In October 2003, public comments from Justice Scalia reportedly led to his recusal from a case challenging the use of the words “under God” in the Pledge of Allegiance. He is believed to have recused himself because of a speech he made in Virginia in January of that year in which he said the appeals court decision rejecting the pledge language demonstrated unwarranted judicial hostility toward religion in public life.

“No one thought the religion clauses were meant to prevent the United States from fostering religious practices,” Justice Scalia said yesterday. “Our history is full of it. … We are not France. We are not Europe.”

Justice Scalia also gave no ground in his campaign to rid American jurisprudence of the influence of foreign judges.”It’s just totally irrelevant what current courts of foreign countries think,” he said.The jurist said judges in other nations are generally even further removed from the public, which has led to judicial fiats, such as the banning of capital punishment by the European Court of Human Rights.

“It’s not as though all the Europeans voted to abolish it. It was judicially imposed and that doesn’t impress me very much,” Justice Scalia said.