This is incorrect. This issue was ginned up by the southern fire-eaters to promote discord between the sections.
The Missouri Compromise had prohibited slavery in Louisiana Territory north of the southern border of MO, in return for admission of MO as a slave state.
30 years later the South reneged on this deal and pushed thru Douglass' "popular sovereignty" to allow for slaves going into the territories.
You will note, however, they did not give up their gain of MO as a slave state from the MO Compromise when they overturned this 30 year precedent.
The question of slavery in the Southern States was not an issue at the beginning of the war, as many believe.
Quite true. I really don't know anybody who claims otherwise. But wars change things. When the colonies went to war in spring of 1775, few wanted independence. A little over a year later the Declaration of Independence passed.
Similarly, in early 1861 the destruction of slavery was not a Union war goal. A little over a year later, summer of 1862, the preliminary Emancipation Proclamation was released.
The southern aristocrats who attended the Constitutional Convention were embarrassed by slavery and were looking for a way to extinguish it. By 1860 that class not only had convinced themselves that it was morally acceptable, they then believed they were doing their slaves a favor.