In The Grapes of Wrath, what role does the bank play? What power do the small farmers have against the banks and the tractors?

In The Grapes of Wrath small farmers are presented as being at the mercy of the banks. The banks and other finance companies own the lion's share of the land on which the farmers work. With the Great Depression in full swing, and with the need for profit ever more urgent, landowners start kicking tenant farmers off their land to satisfy the insatiable appetite for money of the banks back East, portrayed as savage, greedy monsters with their own army of snub-nosed tractors.
The bank is depicted as an especially rapacious, hungry monster that feeds off the hard work of tenant farmers. The financial needs of the banks will always come first. It doesn't matter how long a farming family has occupied a particular plot of land; the bank won't think twice about sending in the tractors to turf out the tenant farmers just to squeeze some extra profit out of the land.
In truth, there's virtually nothing that the tenant farmers can do about it. The main problem is that the decision-making process is entirely subject to the dictates of profit, and so it's impossible to pin the blame for evictions on any one individual. At every stage in the process, individuals—whether they're bankers, landowners, or tractor drivers—are controlled by the need to turn a profit. Among other things, this somewhat absolves them of individual moral responsibility for kicking tenant farmers off their land. It's nothing personal; it's strictly business.