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The Civil War _was_ about "states' rights". Specifically, the claim by some states that they had the right to enslave other human beings and the other states saying "you absolutely do not". The slaveholding states felt so strongly about their "right" to enslave other humans that they went to war over it. This is not complicated and anyone, politician or otherwise, who can't or won't say this explicitly should not be in office or elected to one.
They went to war to INCREASE the numbers of slaves, increase the number of states, double the size of slave states across the "territories" of land stolen from First Nations.
Slavers had already taken Texas from Mexico, and they wanted THE REST OF THE WEST.
The Civil War was a war of EXPANSION, not separation.