Final answer:
The main struggle for admitting Missouri as a new state was maintaining the balance between free and slave states in the Senate. The Missouri Compromise admitted both Missouri as a slave state and Maine as a free state to preserve this balance and established the 36° 30' line as the boundary for future territories.
Step-by-step explanation:
A major reason why Congress struggled to admit Missouri as a new state was because if it were admitted as a slave state, it would disrupt the balance of free and slave states in the Senate. The concern over this imbalance nearly led to a sectional crisis when Missouri petitioned for statehood in 1819, given that it allowed slavery as a territory and planned to continue doing so as a state. This sparked significant debate in Congress, with northern representatives fearing the expansion of slavery and southern representatives concerned about maintaining political power.
Ultimately, the Missouri Compromise was enacted. It solved the immediate problem by admitting Missouri as a slave state and Maine as a free state, maintaining the balance between free and slave states in the Senate. The Compromise also set the Missouri Compromise line at 36° 30' north latitude, which became the dividing line between slavery and freedom for the rest of the Louisiana Purchase territory.