Coons said private and parochial schools are free to teach creationism but that "religious doctrine doesn't belong in our public schools."
"Where in the Constitution is the separation of church and state?" O'Donnell asked him.
When Coons responded that the First Amendment bars Congress from making laws respecting the establishment of religion, O'Donnell asked: "You're telling me that's in the First Amendment?"
Her comments, in a debate aired on radio station WDEL, generated a buzz in the audience.
"You actually audibly heard the crowd gasp," Widener University political scientist Wesley Leckrone said after the debate, adding that it raised questions about O'Donnell's grasp of the Constitution.
Erin Daly, a Widener professor who specializes in constitutional law, said that while there are questions about what counts as government promotion of religion, there is little debate over whether the First Amendment prohibits the federal government from making laws establishing religion.
"She seemed genuinely surprised that the principle of separation of church and state derives from the First Amendment, and I think to many of us in the law school that was a surprise," Daly said. "It's one thing to not know the 17th Amendment or some of the others, but most Americans do know the basics of the First Amendment."
O'Donnell didn't respond to reporters who asked her to clarify her views after the debate.
"Talk about imposing your beliefs on the local schools," she said. "You've just proved how little you know not just about constitutional law but about the theory of evolution."
Coons said her comments show a "fundamental misunderstanding" of the Constitution.
O'Donnell questions separation of church, state