The Fourteenth Amendment (Amendment XIV) to the United States Constitution was adopted on July 9, 1868. The following quotes are examples of the congressional thinking and debates regarding the third section of the 14th at the time.
Senator Edgar Cowan of Pennsylvania attacked Section Three because he was against "to the infliction of punishment of any kind upon anybody unless by fair trial." A Congressman [Benjamin M. Boyer of Pennsylvania] added that Section Three was a criminal "bill of attainder or ex post facto law."
Senator Lyman Trumbull of Illinois, though, rejected a punitive interpretation of Section Three: "[W]ho ever heard of such a proposition...that a bill excluding men from office is a bill of pains and penalties and punishment?" Senator Trumbull stated that the Constitution "declares that no one but a native-born citizen of the United States shall be President . . . Does, then, every person living in this land who does not happen to have been born within its jurisdiction undergo pains and penalties and punishment all his life, because by the Constitution he is ineligible to the Presidency?"
Former Confederates were barred from being seated in the US Congress
The 14th Amendment disqualified these and other former Confederates from elected office. The due process for these men was not a day in court to plead their case. The only process they were due was referring them to the third section of the 14th amendment. The Constitution and its amendments are the law in the land and they cannot be undone in a courtroom, although they are often interpreted.
After the Civil War, pro-Union members of Congress were appalled to find former Confederate officials being elected to federal office. Most egregiously, Georgia elected Alexander Stephens, the onetime vice president of the Confederacy, as a U.S. Senator in 1866. The Senate refused to seat him, and in 1868 Congress adopted Section 3 of the 14th Amendment regarding Disqualification from Holding Office:
Zebulon B. Vance a pre-Civil War Congressman from North Carolina. He served in Confederate army. He was also a North Carolina governor under Confederacy. After the war he was elected U.S. Senator by the North Carolina legislature. The US Congress refused to seat Vance.
Another southerner, John H. Christy, "[V]oluntarily giv[ing] aid, countenance, counsel, and encouragement to persons engaged in armed hostility to the United States." was also elected to Congress. The Georgia Governor refused to commission him, and Congress refused to seat him. (Not long after in 1877, Christy died as a result of a railroad car running over his foot as he disembarked).