Final answer:
The legislation that attempted to maintain a balance between free states and slave states in the senate was the Missouri Compromise in 1820.
Step-by-step explanation:
The legislation that attempted to satisfy the southern demand to maintain a balance in the senate between free states and slave states was the Missouri Compromise in 1820. This compromise allowed Missouri to enter the Union as a slave state while admitting Maine as a free state. It also established a dividing line at 36° 30′, where slavery would be allowed south of the line and prohibited north of it in the Louisiana Purchase lands.