The bottom-line issue is whether the people of a State, which has voted to leave the Union, are engaged in an insurrection.
How one answers that question determines whether one respects individual rights. Nowhere in the Constitution will you find an explicit premption for a State to vote to leave the Union - only r9etb's quotation regarding insurrection seems to apply to the question of whether secession is a legal endeavor. Put another way, nowhere in the Constitution does one find that the people and/or the States have expressly surrendered their Right to opt out of the Union.
After one reads the Ninth and Tenth Amendments to the Constitution, it is clear that the People/State's reserved unto themselves the Right clearly spelled out in the Declaration of Independence to leave the Union. IMHO, it is ludicrous to believe that individuals who penned the Declaration of Independence; agreed with Hobbe's Leviathon opinion of government and fought a war against England for their Individual Rights would then create a government they couldn't legally opt out of when it inevitably became uncontrollable.
Call me crazy, but my time in law school and studying history in undergrad leads me to believe that there is a right to secede - if the people of a State vote to do so.
To me, an insurrection is an illegal attempt to overthrow a government - but seccession, i.e. a vote by the citizens of a state for their state to leave the union, is not an attempt to overthrow the US Government; it is merely an expression of the will of the citizens of that State that they wish to govern themselves in a manner different from that which the US Government is currently operating, and therefore legal under the Constitution and the Bill of Rights.
IMHO.
However, Article VI explicitly says that the individual states, and their elected officials, surrender their sovereignty to the United States.
The secession issue would seem to be decided there: can a state legally seceed once it has pledged itself to remain bound by the Constitution? No, because the Constitution is by definition superior to the anti-Constitutional act of secession.
The 9th and 10th amendments do reserve certain rights and powers to the states and people; however, those rights and powers do not extend to laws and acts that go against the supreme Law of the Land. Secession is in direct conflict with the supreme Law of the Land, and is thus not covered by those two amendments.