The US Supreme Court has heard arguments for removing the words "under God" from the Pledge of Allegiance recited in public schools.
Protesters against any change held a candle-lit vigil outside the court
Atheist Michael Newdow, a California doctor with a law degree, told the court on Wednesday that the government "is supposed to stay out of religion".
But the US solicitor general said the pledge was a "patriotic act".
The case has caused controversy, with observers saying it will be a battle over church-state separation.
Outside the court in Washington DC, protesters on both sides held demonstrations, with supporters of Mr Newdow waving banners saying "Keep church and state separate".
Meanwhile, opponents prayed and displayed signs saying "In God we trust".
Millions of schoolchildren recite the pledge each morning before classes.
Under the current wording, teachers and students say they "pledge allegiance to the flag of the United States of America and to the republic for which it stands, one nation under God, indivisible, with liberty and justice for all".
The words "under God" were added to the original 1892 pledge in 1954, as part of an effort to distinguish the US way of life from the Soviet Union's atheistic communism.
The court has been examining whether the classroom oath crosses America's division of church and state and thereby amounts to an unconstitutional government
endorsement of religion.
Dr Newdow filed the lawsuit because he was unhappy that his nine-year-old daughter had to recite the pledge at her school at Elk Grove in northern California.
He sued her school and won, setting up the landmark appeal before a court that has repeatedly barred school-sponsored prayer from classrooms, playing fields and school ceremonies.
A poll carried out by the Associated Press on the issue suggests that nine out of 10 Americans are in favour of the reference to God remaining in the pledge.
Even Dr Newdow's ex-wife, with whom he shares custody of his daughter, opposes its removal.
Neither does Dr Newdow have the backing of school officials in Elk Grove:
"The argument that 'under God' in the pledge is pushing religion on children is wrong on the law. It's also wrong from a commonsense perspective," he said.