explain how both northerners and southerners believed winning the struggle over the expansion of slavery was the key to preserving their freedoms and to preventing their domination by the other section of the nation.

The northerners thought that every person should have freedom, the southerners didn't believe that the slaves were people.

Explanation:

People in the north at the time believed that if not everyone was free, it wasn't actually a free nation. People in the south saw the slaves more as cash cows, and they thought that taking away the South's main source of income would cripple the American economy.

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.