A decision by the Tasmanian Labor government (then led by Premier Harry Holgate) to dam the Franklin River led Sanders to become a leader of the opposition movement to the proposed dam. Representing the Australian Democrats (then still a new party), he was elected to the Tasmanian House of Assembly as the Member for Denison at a 1980 by-election. This success made him Australia's first parliamentarian ever to be elected on a specifically environmental platform. In parliament, Sanders was a key player in the campaign to save the Franklin River, and successfully moved a motion of no confidence in the Holgate administration during March 1982. This forced an early state election, which Holgate lost.