It was up to each state to determine what injury or oppression they deemed sufficient to secede.
Pres. James Madison, often named the "Father of the Constitution" went into this question at treat length here.
Oh you mean in the 1820s? LOL! Firstly what the STATES agreed to at the time is what is relevant. They were the ones who were actually parties to the contract. What does not matter is the opinion of the Madison 40 years later.
That argument was never proposed in 1788, or at any other time before most or all of our Founders passed away.
And it is specious to the max when applied to 1860, since there was literally nothing then that legally amounted to anything more than unilateral secession at pleasure.
When our Founders talked about "necessary" and "injury or oppression", they were referring back to conditions on their 1776 Declaration, not to just a political faction losing an election.
FLT-bird: "Oh you mean in the 1820s? LOL!
Firstly what the STATES agreed to at the time is what is relevant.
They were the ones who were actually parties to the contract.
What does not matter is the opinion of the Madison 40 years later."
Of course, you don't think the opinions of the Father of the Constitution -- Madison -- matter because you're a Democrat and Democrats hated the Constitution from Day One -- they voted against ratification, supported nullifications, slavery, secession & war against the United States.
So naturally, you disagree with Madison.
But here's an inconvenient fact which you can easily verify yourself: no Founder at any time ever explicitly disagreed with Madison's opinions as expressed here.
Remember: Madison is totally OK with "secession" by mutual consent or by "usurpations or abuses of power justly having that effect."
Note his word "justly".
But Madison opposes unilateral "secession" at pleasure and I have never seen where any Founder disputes that.
Nor have I seen where merely losing an election would be considered anything more than "at pleasure" to our Founders, any Founders.
Nor have you.
Madison was on the committee that wrote Virginia's ratification statement. If he had any objections to the idea Virginia could take back it's sovereignty, he could have demanded the language be changed.
Virginia's ratification clearly states they have the right to resume the powers given up to the federal government.
Even if Madison believed otherwise, he was clearly outvoted by his peers.