Answer: Voting power
Step-by-step explanation:
Abolitionists sought to abolish slavery in the United States and for that to happen they needed a majority in Congress. New States also got to send representatives to Congress and vote on government policy.
It was therefore imperative that a state entered the Union as a free state so that when it sent representatives to Congress (especially the Senate), those reps could vote in favor of abolishing or at least limiting slavery.