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. Her campaign manager, Matt Moran, later issued a statement saying that O'Donnell wasn't questioning the concept of separation of church and state.

"She simply made the point that the phrase appears nowhere in the Constitution," Moran said.

During the exchange, she said Coons' views on creationism showed that he believes in big-government mandates.

"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.

The debate, their third in the past week, was more testy than earlier ones.

O'Donnell began by defending herself for not being able to name a recent Supreme Court decision with which she disagrees at a debate last week. She said she was stumped because she largely agrees with the court's recent decisions under conservative chief justices John Roberts and William Rehnquist.

"I would say this court is on the right track," she said.

The two candidates repeatedly talked over each other, with O'Donnell accusing Coons of caving at one point when he asked the moderator to move on to a new question after a lengthy argument.

"I guess he can't handle it," she said.

O'Donnell, a tea party favorite who stunned the state by winning the GOP primary last month in her third Senate bid in five years, called Coons a liberal "addicted to a culture of waste, fraud and abuse."

Coons, who has held a double-digit lead in recent polls, urged voters to support him as the candidate of substance, with a track record over six years as executive of the state's most populous county. He said O'Donnell's only experience is in "sharpening the partisan divide but not at bridging it."