From The Washington Post:
“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.
Video of it. The quote above is about 6:15 in but you may want to stop at 2:37 also where the audience ends up laughing at O’Donnell.