Daggett had dropped off his car for service and received a loaner from Bridgewater Acura on Saturday.
Campaign spokesman Tom Johnson says Daggett's driver, Ken Argiro, left the weapon under the seat and forgot it. Johnson says Argiro is a retired state trooper and the gun was registered.
Johnson says the dealership called on Monday night and the driver picked up the gun on Tuesday. The incident was first reported by The Star-Ledger of Newark.
The campaign spokesman says Daggett did not realize the gun was in the car.
"It was accidentally left in the loaner car," Daggett told the newspaper. "They called and immediately we took care of it."
Information from: The Star-Ledger, http://www.nj.com/starledger