What was Tom Watson trying to explain to that question in Negro Question in the New South.

He believed the Southern White farmers and Blacks should become natural allies.
Here is a direct quote from the book that seems to answer your question :

The conclusion, then, seems to me to be this: the crushing burdens which now oppress both races in the South will cause each to make an effort to cast them off. They will see a similarity of cause and a similarity of remedy. They will recognize that each should help the other in the work of repealing bad laws and enacting good ones. They will become political allies, and neither can injure the other without weakening both. It will be to the interest of both that each should have justice. And on these broad lines of mutual interest, mutual forbearance, and mutual support the present will be made the stepping-stone to future peace and prosperity.
(Thomas E. Watson, “The Negro Question in the South”)