a prime mover
The person or thing that generates the initial energy or momentum for something to happen. The newly elected congresswoman has been the prime mover in the recent environmental reforms. While there were a confluence of events leading to his removal from office, it was the revelation of his tax evasion over the years that was the prime mover.
Fig. the force that sets something going; someone or something that starts something off. The assistant manager was the prime mover in getting the manager sacked. Discontent with his job was the prime mover in John's deciding to retire early.
The initial source of energy directed toward a goal, someone or something that sets others in motion. For example, Jean was the prime mover in getting us more laboratory space, or Patriotism was the prime mover of the revolution. [Late 1600s]