ASHINGTON, March 24 — Michael A. Newdow stood before the justices of the Supreme Court on Wednesday, pointed to one of the courtroom's two American flags and declared: "I am an atheist. I don't believe in God."

With passion and precision, he then proceeded to argue his own case for why the daily recitation of the Pledge of Allegiance in his daughter's public school classroom violates the Constitution as long as the pledge contains the words "under God."

Dr. Newdow, a nonpracticing lawyer who makes his living as an emergency room doctor, may not win his case. In fact, justices across the ideological spectrum appeared to be searching for reasons he should lose, either on jurisdictional grounds or on the merits. But no one who managed to get a seat in the courtroom is likely ever to forget his spell-binding performance.

That includes the justices, whom Dr. Newdow engaged in repartee that, while never disrespectful, bore a closer resemblance to dinner-table one-upmanship than to formal courtroom discourse. For example, when Dr. Newdow described "under God" as a divisive addition to the pledge, Chief Justice William H. Rehnquist asked him what the vote in Congress had been 50 years ago when the phrase was inserted.

The vote was unanimous, Dr. Newdow said.

"Well, that doesn't sound divisive," the chief justice observed.

Dr. Newdow shot back, "That's only because no atheist can get elected to public office."

The courtroom audience broke into applause, an exceedingly rare event that left the chief justice temporarily nonplussed. He appeared to collect himself for a moment, and then sternly warned the audience that the courtroom would be cleared "if there's any more clapping."

Earlier, Dr. Newdow responded to Justice Stephen G. Breyer's suggestion that "under God" had acquired such a broad meaning and "civic context" that "it's meant to include virtually everybody, and the few whom it doesn't include don't have to take the pledge."

"I don't think that I can include `under God' to mean `no God,' " Dr. Newdow replied. "I deny the existence of God." He added, "Government needs to stay out of this business altogether."

The current Pledge of Allegiance was defended by Solicitor General Theodore B. Olson and by Terence J. Cassidy, the lawyer for the Elk Grove school district in California where Dr. Newdow's daughter attends elementary school.

Both lawyers were appealing a decision won by Dr. Newdow in the United States Court of Appeals for the Ninth Circuit, in San Francisco. That court ruled last year that the addition of "under God" turned the pledge into a "profession of religious belief" and made it constitutionally unsuitable for daily recitation in the public schools. Congress added the phrase at the height of the cold war in an effort to distinguish the American system from "Godless Communism."

One justice, Antonin Scalia, was sharply critical of the appeals court's ruling in a speech he gave before the case reached the Supreme Court. As a result, Dr. Newdow asked Justice Scalia to recuse himself, and the justice complied, without comment, when the court accepted the case in October. His absence raises the possibility of a 4-to-4 tie, which would automatically affirm the Ninth Circuit's ruling without setting a binding precedent elsewhere.

Solicitor General Olson told the justices that the appeals court misunderstood the pledge. The phrase "under God" did not place the pledge in the category of religious expressions that the Supreme Court has found unconstitutional, he said, for example "state-sponsored prayers, religious rituals or ceremonies, or the requirement of teaching or not teaching a religious doctrine."

Rather, Mr. Olson said, "under God" was one of various "civic and ceremonial acknowledgments of the indisputable historical fact that caused the framers of our Constitution and the signers of the Declaration of Independence to say that they had the right to revolt and start a new country." He said the framers believed "that God gave them the right to declare their independence when the king has not been living up to the unalienable principles given to them by God."

That description of the pledge appeared to gain little traction as the argument proceeded. "I do assume that if you read the pledge carefully, the reference to `under God' means something more than a mere description of how somebody else once thought," Justice David H. Souter said to Dr. Newdow moments later.

Justice Souter's question for Dr. Newdow was whether, even assuming that schoolchildren were being asked "as a technical matter" to make a personal religious affirmation, the recitation had become in practice "so tepid, so diluted, so far, let's say, from a compulsory prayer that in fact it should be, in effect, beneath the constitutional radar." Was it the case, Justice Souter asked, that by "the way we live and think and work in schools and in civic society in which the pledge is made, that whatever is distinctively religious as an affirmation is simply lost?"

Dr. Newdow replied: "That is a view that you may choose to take and the majority of Americans may choose to take. But it's not the view I take, and when I see the flag and I think of pledging allegiance, it's like I'm getting slapped in the face every time, bam, you know, `this is a nation under God, your religious belief system is wrong.' "

Before the justices can decide the merits of the case, Elk Grove Unified School District v. Newdow, No. 02-1624, they must resolve doubts about whether Dr. Newdow had standing to bring his lawsuit, either on his own behalf or on behalf of his daughter, who is now 9 years old. A court does not have jurisdiction in the absence of a plaintiff with standing.

Dr. Newdow was never married to the child's mother, Sandra Banning, who has custody and has told the court in a brief filed by Kenneth W. Starr, the former independent counsel, that she is giving her daughter a religious upbringing and wants her to say the pledge with "under God." The justices spent about half of the one-hour argument posing questions about standing and sparring with Dr. Newdow on the subject.

It remained unclear whether Dr. Newdow persuaded them, but he was obviously prepared for the argument. He told the justices, "I am saying I as her father have a right to know that when she goes into the public schools she's not going to be told every morning to stand up, put her hand over her heart, and say your father is wrong, which is what she's told every morning."

"Well, she does have a right not to participate," Justice Sandra Day O'Connor observed.

In 1943, 11 years before Congress added "under God," the court ruled that no one could be compelled to say the Pledge of Allegiance. That case was brought by Jehovah's Witnesses, whose religion forbids saluting the flag.

Dr. Newdow said opting out was a "huge imposition to put on a small child."

He continued: "Government is doing this to my child. They're putting her in a milieu where she says, `Hey, the government is saying that there is a God and my dad says no,' and that's an injury to me."

Dr. Newdow, 50, often spoke very rapidly but never appeared to lose his footing during the 30 minutes the court gave him. He managed a trick that far more experienced lawyers rarely accomplish: to bring the argument to a symmetrical and seemingly unhurried ending just as the red light comes on.

"There's a principle here," he told the justices in his closing moments, "and I'm hoping the court will uphold this principle so that we can finally go back and have every American want to stand up, face the flag, place their hand over their heart and pledge to one nation, indivisible, not divided by religion, with liberty and justice for all."