I'm going to answer this without thinking of the Civil War or today's issues; only by the definition of the words [the United States] "Union" and [the U.S.] "Constitution" and one more "States".

Preserving the Union is more important than preserving the Constitution in relation to the protection of States. The Constitution would not exist without the Union. The Union would not exist without the States. The Union was created to protect the States by mutual cooperation. The Constitution was created by the States to establish a government between sovereign States for the purpose of Union.

Preserving the sovereignty of each State is more important than either the Union or the Constitution, for from this sovereignty is derived the Rights of the People.

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David Upton