Answer:
The admission of Missouri as a state threatened the Senate balance between the free state and the slaveholding states because the slaveholding state fear that if they we outnumbered in the congress representation, might result into them not having the required power to protect and defend their trade and properties.
Step-by-step explanation:
The Missouri compromise of 1820 was targeted at balancing representation and power between the free states and the slaveholding states. The senate admitted the Missouri as a slaveholding state while Maine was admitted as a free state.
The debate that threatened the Senate was when Missouri applied to become part of the union and also allow slave within its territory lead to arguement and counter argument which divided the congress into pro slave and anti slave divisions that went on for almost a year but the application of Maine as a free state calm down nerves that resulted into admittion of Maine as a free state and Missoiuri as slavesholding state to balance the equation.