In The Grapes Of Wrath, what power do the small farmers have against the banks and the tractors?

The small farmers are tenant farmers, they do not own the land where they live and work.  So, technically, they are powerless to fight against the tractors and the banks.

The owners of the land evict, or throw the tenants off the land, in an effort to survive themselves.  The owners must pay their mortgages to the banks, and in the absence of payment the bank takes back the property.

The small farmers have no choice but to leave the land because the tractors, which are driven by neighbors of the farmers desperate for money to support their own families, are determined to clear the land of the run down houses of the tenants.

"When the farmer threatened to shoot him, the boy said it would do no good; another tractor driver would be pushing the house down even before the farmer was surely hanged for shooting him."

Without ownership rights, the small farmers must leave.  The situation that Steinbeck describes is one of total deprivation.  The owners are responsible to the banks for their mortgages, the tenants, who would share in the farm's profits have no money, the owners have no money, so there is nothing that the small farmers can do except leave the barren land.