James McHaney, 28, was arrested Friday afternoon when he arrived to meet the "teenager" he had been communicating with on the Internet, which TheSmokingGun Web site first reported Monday.
Court records said McHaney was charged "with using a computer to arrange a sexual encounter with an informant posing as a boy" under the Attempted Child Exploitation Act, The Seattle Times reported Tuesday.
McHaney worked as a scheduler for Sen. Maria Cantwell, D-Wash. Her chief of staff, Michael Meehan, issued a statement, saying after being informed of the FBI arrest, "the employee was immediately fired."
Before working for Cantwell, the Times said McHaney worked for the 2004 John Kerry presidential campaign and earlier as an aide to former Rep. Dick Gephardt, D-Mo.