What was the last failed effort to prevent the Civil War?

The Crittenden Compromise or Crittenden Plan was the nation's last effort to prevent the civil war.

Defininition of Crittenden Compromise: A series of constitutional amendments proposed in Congress in 1860 to serve as a compromise between proslavery and antislavery factions, one of which would have permitted slavery in the territories south but not north of latitude 36°30′N. Also called Crittenden Plan.