What key argument did Lincoln present against slavery?

Lincoln's argument against slavery fundamentally revolved around the idea that it violates basic human rights. He believed that slavery was fundamentally incompatible with the principles of democracy and liberty that the United States was founded upon. In his speeches and writings, he often emphasized the moral dimension of slavery, arguing that it was an institution that dehumanized individuals and denied them their inherent rights as human beings.

Lincoln pressed that any government based on the principles of equality and freedom could not simultaneously tolerate an institution that enslaves a segment of its population. His famous statements during debates, such as those with Stephen A. Douglas, underscored his commitment to the notion that all individuals have the right to freedom and self-determination.

While societal division and economic arguments were also considerations in the broader discussions around slavery during his presidency, Lincoln's core moral objection was rooted in the violation of basic human rights, which ultimately shaped his policies and the direction of the nation during the Civil War.