The case turns on the meaning of Section 3 of the 14th Amendment, ratified after the Civil War, which bars those who had taken an oath “to support the Constitution of the United States” from holding office if they then “shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof.”
And then there’s the historical folks squabbling about how you can’t change the constitution. The country changes but we’re all still working on late 1700s language, plus amendments.