foot soldier

noun2

1. •
   A soldier who fights on foot; an infantryman
2. •
   (figurative) A rank and file member of an institution, or supporter of a cause.

   • We are rightfully wary of straight Leftists who didn't give a damn for our struggle until they saw we had build ^([sic]) on our own, and then tried to recruit foot-soldiers from our ranks. — 1980