Answer:
The correct answer is: Northerners feared it would lead to the expansion of slavery.
Step-by-step explanation:
The northerners believed that winning the struggle over the expansion of slavery was key to preserving their freedom because, if they could stop the practice and expansion of slavery into new regions where it was not being practiced, then they could prevent those regions from joining the slave-holding southerners who were interested in having more people in congress, and increasing their power in the country, and over the constitution: if the northerners could prevent the expansion of slavery, then they could influence national politics. On the other hand, the southerners believed that winning the struggle over the expansion of slavery was key to preserving their freedom because, if they could expand slavery into new regions, especially the northern region, then they could have more people in congress to protect their interest, make them become politically stronger, and have greater influence in national politics.