In the mid-nineteenth century, what did most Americans believe the Constitution said about who had the power to decide about the existence of slavery in the United States?

Slavery itself was never widespread in the North, though many of the region's businessmen grew rich on the slave trade and investments in southern plantations. Between 1774 and 1804, all of the northern states abolished slavery, but the so-called "peculiar institution" of slavery remained absolutely vital to the South.

Explanation:

U. S. Congress outlawed the African slave trade in 1808, the domestic trade flourished, and the enslaved population in the U. S. nearly tripled over the next 50 years. By 1860 it had reached nearly 4 million, with more than half living in the cotton-producing states of the South.

Enslaved people in the antebellum South constituted about one-third of the southern population. Most lived on large plantations or small farms; many masters owned fewer than 50 enslaved people.

Answer and Explanation:

In the mid-nineteenth century, most Americans (particularly the Southerners) believe the Constitution said that the individual states have power over the federal government to decide about the existence of slavery in the United States.

Not until Abraham Lincoln and the Northerners stepped in and insisted that the Union's victory brought about the US, so the consistution was amended and the federal government was given the authority to decide the existence of slavery later on.