Eli Whitney's invention of the cotton gin and new farmlands in the west (westward expansion) led to

Increasing demand for slaves in the years leading up to the Civil War.

Eli Whitney's invention of the cotton gin and new farmland expansion to the west led to an increased demand for slaves to work the fields for cotton production. With the cotton gin, cotton could be processed more quickly, which meant that more slaves were needed to pick the cotton. Along with that, the westward expansion meant even more cotton fields to be worked.