Robert Hurt's congressional campaign spokesman Sean Harrison says the powder was found in an envelope Hurt opened Friday afternoon.
Pittsylvania County Sheriff Mike Taylor said results of tests conducted on the substance will be available in two to three days. Taylor said the U.S. Postal Service is leading the investigation.
Harrison said Hurt followed decontamination procedures after opening the envelope, including washing thoroughly and changing his clothes, before going home to his family Friday evening.
Hurt is trying to move from Virginia's state Senate to the U.S. House of Representatives.
He is challenging first-term Democrat Tom Perriello for Virginia's 5th Congressional District seat.
The race is one of the nation's most closely watched midterm House races.