Why could Southern politicians be considered over-optimistic in accepting this deal? What might they have been hoping for in the future if they supported the spread of slavery?

They ensured a huge free-state like California while giving the regions a chance to decide in favor of themselves.

They trusted the regions would cast a ballot to permit subjugation.

They trusted that if the regions cast a ballot to permit subjugation they would hold an enormous populace and various states the regions made up more land than California.