I used to be more confused on this historical point. After many years of study, it became clear. Lincoln hated slavery, but his main goal was to preserve the Union of States. The states decided they could go against The Declaration of Independence in order to hold slaves. "All men are created equal". They they wanted to use the Constitution and so forth to justify this violation of the foundational document mostly due to greed since slavery was lucrative. Hence, they said the Federal Government has no power to enforce ultimately its founding document. This was wrong since the entire purpose of the Federal Government is to enforce the framework of the Declaration of Independence and secure its continual functioning.

Hence, slavery was the "breaking issue" due to hypocrisy and a war had been only narrowly avoided before Lincoln became president.