[DoodleDawg]

Madison believed that unanimous consent would be required for even one state to secede because that act of secession would dissolve the Union and abrogate the contract.

I know that Madison said that a proper secession required the consent of the other states but I'm not aware where he said it required unanimous consent.

[Publius]

It's a basic principle of contract law. When the Federalists of New England opposed the Louisiana Purchase in 1803, they argued that the new territory would so change the nature of the Union that unanimous consent would be required, not just a constitutional amendment or an act of Congress. In this, they were echoing Madison. Jefferson rejected that argument, instead taking up the argument of his enemy Hamilton that the acquisition of new territory was simply a function of nationhood.

Lincoln took a more moderate position. As a practicing lawyer, he understood that the instrument through the Union would be dissolve and reformed would be a constitutional amendment. Part 1 would dissolve the Union, Part 2 would reform the Union with those states that chose to remain, and Part 3 would be some boilerplate authorizing Congress to enact legislation supporting all this. The amendment could be proposed via Congress or an Amendments Convention, and ratification would be via state legislatures or state ratifying conventions, as Congress so chose under the terms of Article V. For ratification, three-fourths of the states, not unanimous consent, would be sufficient. Lincoln asked the Southern states to return their representatives and senators to Congress to negotiate an amicable divorce. The events at Fort Sumter changed the terms of the discussion. It would now be negotiated in the Court of Arms.

I would imagine that Texas could leave if a constitutional amendment, ratified by three-fourths of the states, permitted an amicable divorce.