It’s Candid Camera for the Internet age, a lethally effective political tool that Mr. O’Keefe has helped pioneer between college and graduate-school studies. He has lampooned liberals by inviting them to become pen pals of imprisoned terrorists, and, more darkly, recorded Planned Parenthood staff members agreeing that he can designate his donation exclusively to the abortion of black babies.

Image James E. OKeefe III, on Fox News discussing his hidden-camera videos with Acorn workers. Credit... Fox News

But never has his work had anything like the impact of the Acorn exposé, conducted by Mr. O’Keefe and a friend he met through Facebook, 20-year-old Hannah Giles. Their travels through various offices of Acorn, the national community organizing group, caught its low-level employees in five cities sounding eager to assist with tax evasion, human smuggling and child prostitution.

What began as a firestorm on conservative talk shows spread across the news media and to both parties in Congress, as Mr. O’Keefe and Ms. Giles unveiled videos of more cities where Acorn workers had misbehaved. Jon Stewart, whose “Daily Show” on Comedy Central leans to the left, highlighted the videos, and a House proposal to cut off all federal money to Acorn passed Thursday by a vote of 345 to 75.

In a telephone interview on Thursday night — when he was editing still more Acorn footage — Mr. O’Keefe said that when he accepted Ms. Giles’s idea that they take on Acorn, “I thought we’d get some snippets” worth posting to the Web. “I’m a skinny nerd, the least convincing pimp in the world,” he said.

Instead, a succession of Acorn workers advised the pair on how to smuggle Salvadoran girls into the country, falsify a loan application to buy a house for use as a brothel and even claim the under-age prostitutes as dependents for tax purposes.