1
general::
The impetus, power, or energy behind something in motion, as in He was clearly the driving force in the new administration. This term transfers the force that sets in motion an engine or vehicle to other enterprises. Ralph Waldo Emerson was among the first to use it figuratively (English Traits, 1856): "The ability of its journals is the driving force."
American Heritage Idioms
2
general::
Phrase(s): *the driving force (behind someone or something)
the person or a thing that motivates or directs someone or something. (*Typically: be ~; become ~; serve as ~.) • Making money is the driving force behind most businesses. • Ambition is the driving force behind Tom. • Love can also be the driving force.
McGrawhill's American Idioms And Phrasal Verbs