Their tactics and aims are somewhat different. Mr. O’Keefe often operates with undercover cameras, sending in people playing characters like a pregnant woman (to Planned Parenthood) or a prostitute (to Acorn), to try to entrap their workers into discussing improper or potentially illegal activities. His participants don’t realize they are being filmed, which is not the case with Mr. Cohen’s work.

Mr. O’Keefe also seems more interested in tearing down institutions than merely embarrassing them, as Mr. Koppel noted in a phone interview.

“On the one hand, I don’t want to be perceived as approving of what Mr. O’Keefe has done but at least he’s doing it for an ideological end,” Mr. Koppel said. “There’s a purpose to it other than getting a few giggles.”

Mr. O’Keefe’s Acorn operation essentially destroyed that group as lawmakers and government agencies began starving it of funding. The gotcha videos on Planned Parenthood by his and other groups opposed to abortion have made the organization a constant target in Congress. He even attempted to trick The Washington Post into publishing a hoax story about Mr. Moore, apparently hoping to shame the paper into a retraction, but The Post got wise to the attack and exposed it first.