Oh for God's sake, do you people care to even read the document?
United States Constitution said:
To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions
So if insurrection is legal, why is the federal government empowered to suppress it, hmmm?
United States Constitution said:
The Privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it.
Again, if rebellion is legal, why the injunction against it?
United States Constitution said:
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Confederacy is out.
United States Constitution said:
No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
As is keeping all those troops they were keeping. Following so far?
United States Constitution said:
The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States
So, who's running the Army again? Ain't the states.
United States Constitution said:
Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.
Now then, who was levying war against the United States? Oh, that's right...
United States Constitution said:
The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.
This one's the kicker. When taken in the context of the Supremacy Clause, we see that the states cannot violate the territorial sovereignty of the United States. Secession is such a violation. Here is that Clause, in case you've forgotten:
United States Constitution said:
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.
There's your constitutional argument for the unconstitutionality of secession. If need be, I can bring forward multiple Federalist papers and letters from the members of the Constitutional Convention clearly spelling out that allowing secession was not their intent. As I pointed out above, James Madison thought the idea was so preposterous that it was a wonder that he should have to point out why it's wrong. He also pointed out that the people agitating for the "states' right" of secession at the time (the Nullification Crisis) would just as surely object to the same principle turned on it's head; a majority of the other states voting to kick one state out. Secessionism was never a stand on principle; it was always merely a political tool of blackmail to force the federal government to pander to the interests of the region threatening it.