Question

How were northern lawmakers convinced to allow Missouri into the union as a slave state under the Missouri Compromise?

Answer 1

the way that northern lawmakers were convinced to alow missouri into the union as a slave state under the missouri compromis was : maine was added to the united states as a free state. 
 After prolonged heated discussion, both northern and southern side decided to find a middle ground in the form of missouri compromise. one of the compromise is the addition of Maine (which previously belonged to french) to be administered as a province of massachusets