The Northeast was the region most opposed to the statehood of Missouri as a slave state due to

The Northeast was worried upsetting the balance of power in the Senate.

The controversy over whether Missouri should be admitted as a slave state, resulted in the Missouri Compromise of 1820, which specified that the purchase of the Louisiana Territory north of the 36 ° 30 'parallel, describing most of the border South of Missouri, would be organized as free states and territories south of that line would be reserved for the organization as slave states. As part of the compromise, the admission of Maine (1820) as a free state was an insurance to balance the admission of Missouri as a slave state (1820).

The correct answer is due to the change of all the power that the Senate had in that time. Back in the year of 1818, Missouri wanted to apply to the united states government for statehood but this created an issue for all the country because there were some states which had slaves and others did not have them. they argued about two years because they wanted to decide which state Missouri would be if it allows slaves or not.