NEW ORLEANS  Federal officials charged four men on Tuesday with plotting to tamper with the telephone system in the New Orleans office of Senator Mary Landrieu, Democrat of Louisiana. One of the men was a conservative activist who gained fame last year by secretly recording members of the community group Acorn giving him advice on how to set up a brothel.

All four of the men arrested Monday in New Orleans, each in his mid-20s, were charged with entering federal property under false pretenses with the intent of committing a felony, according to the United States Attorney’s Office for the Eastern District of Louisiana. They appeared in court on Tuesday, and a preliminary hearing was scheduled for Feb. 12.

If convicted, the four would face sentences ranging from a fine to 10 years in prison.

The political activist was James O’Keefe, 25, who has gained renown in conservative circles by poking fun at the left through pranks and undercover video. In the Acorn videos, Mr. O’Keefe and an associate, Hannah Giles, posing as a pimp and a prostitute, secretly filmed themselves seeking and receiving financial advice for a brothel from Acorn workers.

The film damaged Acorn’s reputation, and prompted a move by Congress to cut off some of its federal money. The group has sued Mr. O’Keefe and Ms. Giles, saying the secret recordings were illegal, and late Tuesday, Acorn’s chief executive, Bertha Lewis, said the arrest was further evidence of Mr. O’Keefe’s “disregard for the law in pursuit of his extremist agenda.”