This is AI explanation, but I tend to agree:
Before the Civil War, the South held a disproportionate amount of power in the federal government—a phenomenon Northerners called the "Slave Power Conspiracy."
Because of this extra weight, the South dominated the presidency, the Supreme Court, and Congress for decades.
Northerners realized that if slavery kept expanding into new Western territories, new slave states would be admitted to the Union. This would give the South permanent control of the federal government, leaving the North politically powerless.
