What is meant by a "growth of slave power" as a cause of the war?

The meaning of "growth of slave power" means that

The political power of slavery supporters grew.

Explanation:

To understand this question we need to analyze the context. First of all, this was a cause of the war because slavery supporters had managed to influence heavily politicians and social figures with influence to support their claims in slavery. There was almost always the same number of slaver because the plantations were passed as an inheritance. So they didn't grow in number. Also, the slaves were not trained for war so that's not the case. However the number of slaves did grow, but solely a number of the population didn't mean any power because they were at their maximum only 4 million against free people o 60 million.