The theme of power versus powerlessness is established early in the novel. George and Lennie display this dynamic with George acting as the dominant and controlling figure early. We see this when George demands that Lennie give up the dead mouse he has been carrying around.
George’s hand remained outstretched imperiously. Slowly, like a terrier who doesn’t want to bring a ball to its master, Lennie approached, drew back, approached again. George snapped his fingers sharply, and at the sound Lennie laid the mouse in his hand.
Soon after this moment, Lennie asserts his own power in the relationship - a moral and literally pathetic power - as he suggests that he could run off into the woods if George does not want hiim around anymore.
The power dynamic between the "haves" and the "have nots" is expressed throughout the novel. At one point, Lennie sums up the the intimidation he feels from the owner's son, Curley, and his wife, saying:
Lennie cried out suddenly—“I don’t like this place, George. This ain’t no good place. I wanna get outta here.”
The fact that Lennie and George would want to escape the situation suggests that their ability to assert themselves within it is significantly limited. Yet they must stay because they "gotta raise a stake". They have no material/financial power in the world and so must endure the risks and the labor of the ranch.