How did a high enslaved population in the southern states threaten the people of the northern states?

For the answer to the question above, this refers to Three-Fifth Compromise where the South used the number of slaves in its territories to increase its representation.  It gave the South more power by using the slave population to its advantage.  This would threaten the balance of power, particularly to the Northern States.

Lowered costs because it caused more competition and increased the size of the economy as a whole.