Final answer:
The Crittenden Compromise proposed constitutional amendments to protect and possibly expand slavery into new territories south of the 36°30' line but was rejected by Lincoln and the Republican Party as it went against their anti-slavery expansion policies.
Step-by-step explanation:
The Crittenden Compromise was a last-ditch effort to prevent the American Civil War by Senator John J. Crittenden of Kentucky. It proposed constitutional amendments to guarantee the existence of slavery in federal territories south of the 36°30' line, from the Missouri Compromise. The compromise also sought to extend this line to the Pacific Ocean, ensuring that new territories could opt for slavery. Moreover, it aimed to protect the interstate slave trade and proposed that Congress could not interfere with slavery where it already existed.
President-elect Abraham Lincoln and the Republican Party, however, rejected the Crittenden Compromise, as it contradicted their core principle of preventing the expansion of slavery into new territories. This opposition was crucial as support from the incoming administration was necessary for any compromise to have a chance at success. As a result, the proposal failed, and it neither prevented the secession of Southern states nor stopped the onset of the Civil War.