[Poohbah]

The Constitution, IMNHO, does make some allowance secession, but not on a unilateral basis by a disgruntled state. Essentially, the Constitution gives Congress authority to determine the territorial extent of the United States.

[justshutupandtakeit]

I do not believe there is one iota of evidence that the constitution allows either secession or explusion of a state without an amendment. Nor is there any evidence any of the principle founders believed it to be legal. Not Washington, Hamilton, Adams, Jay, Marshall, Madison or even Jefferson. Washington's Farewell Address was aimed at those who would try and spread the idea of disunion.

Speaking of Jefferson, while I do not consider him in any way an expert on the constitution, he did not believe the document gave him the right to buy Louisiana. Madison had to convince him to put a sock in it to get the deal done by Congress.