Slavery had always been a contentious issue in the United States. But in the 1840s and 50s, with the rapid expansion of the country into new territories such as Kansas, Texas, and California, the so-called "peculiar institution" generated even greater conflict between slavery's supporters and opponents, between slave states and free.
Opponents of slavery were concerned that as the country expanded, slavery would also expand into the new territories. Naturally, they opposed this dark prospect on moral grounds. More slavery would mean more of the attendant evils that the institution brought with it. The suffering and cruelty that were essential components of slavery would no longer be confined to the South, but would spread like a cancer to the West and Midwest, and this was a prospect too horrible for the opponents of slavery to contemplate.
But what really made slavery such a contentious issue in the 1840s and 50s was the very real prospect that the expansion of slavery into the new territories would give the slave interest more power within the American political system.
The slave interest was already very powerful, and so long as that was the case, it would be hard to restrict slavery in the United States, let alone abolish it. The prospect of pro-slavery politicians being elected from the new territories when they became states would make the abolition of slavery virtually impossible, so abolitionists fought hard to make sure that the geographical expansion of the United States would not also lead to the expansion of slavery.