What did Jefferson mean when he wrote of a geographical line marking a moral and political principle?

What did Jefferson mean when he wrote of a geographical line marking a moral and political principle? Why did he believe that it was a threat to the union of the states? What was Jefferson’s preferred way to end slavery and free slaves? Why did he think it improbable, if not impossible?

Did he offer a states-rights argument for determining whether and where slavery continued? Explain. Why did he mourn that the passions of the current generation were going to nullify the sacrifices of the “generation of 1776”?