1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs