Legal Question in Constitutional Law in Maryland

As to secession, no. I'm not sure what you mean by "nullification".

Article V of the Constitution says the states may call a new Constitutional convention as an alternative to the normal amendment process, bypassing Congress. This has never been done, but it's allowed. The legislatures of two-thirds of the states -- which currently means 34 of them -- would have to call for the convention.

The Constitution says only that such a convention could propose new amendments. But the delegates could try to propose an entirely new -- and radically different -- constitution. The key words there are "try" and "propose". It's not clear that they would have the authority to do something that radical. And even if they do, their proposal would still have to be ratified by the states before it would take effect. That would ordinarily require the approval of three-fourths of the states' legislatures-- which currently means 38 of them.

Congress has the authority to require the approval of local conventions in three-fourths of the states instead of the legislatures. But there's no mixing and matching of legislative approval in some states and conventions in others.

Only one part of the Constitution that can't be changed by amendments or by conventions: the guarantee of equal representation in the Senate. A state can agree to accept reduced representation, but it can't be compelled to do so. The rest of the Constitution can be modified.