The Democrats had Abraham Lincoln removed from ten states' ballots. After Lincoln won the election despite the Democrats' election interference, they started an insurrectionist civil war. A war they lost badly.
That must have stung. Afterwards the southern Democrats began to send "unrepentant" former Confederates (such as the Confederacy's former vice president, Alexander H. Stephens) to Washington as senators and representatives. That is why the third section was added to the Fourteenth Amendment to the Constitution. It reads thus:
"Section 3. No person shall be a Senator or Representative in Congress, or elector of President and Vice President, or hold any office, civil or military, under the United States, or under any State, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any State legislature, or as an executive or judicial officer of any State, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof. But Congress may, by a vote of two-thirds of each House, remove such disability."
Now the only the only time it has actually been invoked is:
"Between Reconstruction and 2021, Section 3 was invoked only once: it was used to block Socialist Party of America member Victor L. Berger of Wisconsin—convicted of violating the Espionage Act for opposing US entry into World War I—from assuming his seat in the House of Representatives in 1919 and 1920."
Link for above quotes: Disqualification from office for insurrection or rebellion
So, historically, the only time it was used was then person was actually convicted of spying which is pretty clearly insurrectionist behavior. He did have his spying conviction overturned and then went on to serve several terms.
I ask. Did Donald Trump start a Civil War? Has he even been charged or convicted of insurrection or rebellion? No. But I guess questioning the outcome of an (s)election now qualifies as such. So, here is their case:
"A long shot legal bid in multiple US states to disqualify Donald Trump from the 2024 US presidential ballot has led to him being kicked off the ballot in Colorado and Maine.
"The strategy involves trying to block Mr Trump from the primary ballot by invoking a rarely used provision of the US Constitution - Section 3 of the 14th Amendment - that bars those who have "engaged in insurrection or rebellion" against the country from holding federal office.
"It was the first time that Section 3 of the 14th Amendment was used to disqualify a presidential candidate."