On November 2, 1920, something happened that had never happened before in history and is never likely to happen again: a federal prisoner received nearly 1 million write-in votes for president, including 22% of the New York vote.
This wasn't just any prisoner. This was a legendary union leader who was now a political prisoner.
This was Eugene Debs.

"while there is a lower class, I am in it, and while there is a criminal element I am of it, and while there is a soul in prison, I am not free."
- Eugene Debs at his sentencing

This was the fifth time that Debs had run for president. In 1912 Debs ran as a candidate for the Socialist Party and collected 6% of votes.
It was on June 16, 1918, when Debs dared to speak out against the war, that his problems began.

These are the gentry who are today wrapped up in the American flag, who shout their claim from the housetops that they are the only patriots, and who have their magnifying glasses in hand, scanning the country for evidence of disloyalty, eager to apply the brand of treason to the men who dare to even whisper their opposition to Junker rule in the United Sates. No wonder Sam Johnson declared that “patriotism is the last refuge of the scoundrel.” He must have had this Wall Street gentry in mind, or at least their prototypes, for in every age it has been the tyrant, the oppressor and the exploiter who has wrapped himself in the cloak of patriotism, or religion, or both to deceive and overawe the people.
...
Every solitary one of these aristocratic conspirators and would-be murderers claims to be an arch-patriot; every one of them insists that the war is being waged to make the world safe for democracy. What humbug! What rot! What false pretense! These autocrats, these tyrants, these red-handed robbers and murderers, the “patriots,” while the men who have the courage to stand face to face with them, speak the truth, and fight for their exploited victims—they are the disloyalists and traitors. If this be true, I want to take my place side by side with the traitors in this fight.
...
Wars throughout history have been waged for conquest and plunder. In the Middle Ages when the feudal lords who inhabited the castles whose towers may still be seen along the Rhine concluded to enlarge their domains, to increase their power, their prestige and their wealth they declared war upon one another. But they themselves did not go to war any more than the modern feudal lords, the barons of Wall Street go to war. The feudal barons of the Middle Ages, the economic predecessors of the capitalists of our day, declared all wars. And their miserable serfs fought all the battles. The poor, ignorant serfs had been taught to revere their masters; to believe that when their masters declared war upon one another, it was their patriotic duty to fall upon one another and to cut one another’s throats for the profit and glory of the lords and barons who held them in contempt. And that is war in a nutshell. The master class has always declared the wars; the subject class has always fought the battles. The master class has had all to gain and nothing to lose, while the subject class has had nothing to gain and all to lose—especially their lives.

It is for this speech that he was thrown into jail. The law he was supposed to have broken by saying these words was the Espionage Act of 1917. Specifically Eugene Debs was accused of "obstructing recruiting".
Of course everything that Debs said in Canton was true, but that didn't stop President Wilson from calling him a "traitor to his country". At the trial it was revealed that Debs made his speech only after the Declaration of Independence had been read.
At the trial Debs chose to defend himself. It was a purely political trial and Debs approached it that way.

He did not offer any argument upon the evidence. He did not once employ his gift of ironic confutation, which might have exposed weak points in the case of the prosecution. He did not even condescend, as his attorneys urged him, to present the outline of a legal argument upon which a juryman so disposed might rest his emotional desire to acquit him. With a very genial — and privately almost uproarious — scorn for the whole legal apparatus in which they were trying to tie up his clear-motived intelligence, he simply remained high up in the region of truth and noble feeling, where he lives, and compelled the court to come up there and listen to him or not listen at all.

Debs was convicted and sentenced to 10 years in jail. He was also disenfranchised for life.
Debs went to prison on April 13, 1919. He case was appealed all the way to the Supreme Court, where it was upheld.

Debs' health deteriorated in the poor prison conditions of the time. He was 65 years old and it was unlikely he would ever live to serve out his sentence. Attorney General A. Mitchell Palmer recommended clemency, but President Wilson vowed "Never!" when receiving the petition. Wilson wrote 'Denied'across it.
Wilson was supported by the American Legion, which had launched a "Keep Debs in Jail" campaign.

The 1920 Socialist Party Convention (May 4-14) was conducted in the atmosphere of the first Red Scare and the Palmer Raids. No official stenographic report from the convention exists due to the strained financial conditions of the party. However, there was unofficial recordings, such as the keynote speech by Morris Hilquit.

If there remain any large sections of workers who put their naive faith in old-party messiahs, Woodrow Wilson must have effectively destroyed their faith. For be it remembered that in 1916, Woodrow Wilson ran as a “radical.” He promised practically socialism
through the shortcut of the Democratic Party.
...
Wilson, the pacifist, drew us into the world’s most frightful war.
Wilson, the anti-militarist, imposed conscription upon the country in war, and urged a large standing army and a huge navy in peace.
Wilson, the democrat, arrogated to himself autocratic powers grossly inconsistent with a republican form of government.
Wilson, the liberal, revived the medieval institutions of the inquisition of speech, though, and conscience, His administration suppressed or tired to
suppress radical publications, raided houses and meeting places of political opponents, destroyed their property, and assaulted their persons.
Wilson, the apostle of the “new freedom,” infested the country with stool pigeons, spies, and agents provocateurs, and filled the jails with political prisoners.
Wilson, the champion of labor, restored involuntary servitude in the mines and on the railroads.
Wilson, the idealist and humanitarian, has inaugurated a reign of intellectual obscurantism, moral terrorism, and political reaction the like of which this country has never known before.

The convention was highly contentious with stark divisions between the moderate socialists and the more vocal communists. The moderates won the day, partly because Debs outright refused to consider himself a communist.

"I am not a Communist; I am a Socialist. My party is not a Communist party; it is a Socialist party. We cannot go in."
- Eugene Debs, 1920

The Socialist Party platform was widely criticized by the mainstream media for its radical agenda (when it was acknowledged at all). The platform called for a) minimum wage, b) and end to child labor, and c) rights for black Americans. The platform also included proposals for improving working conditions, housing and welfare legislation, and increasing the number of people who could vote.

Debs, of course, couldn't conduct a normal political campaign. It was limited to making the occasional speeches to sympathetic reporters from inside prison walls. Despite his declining health and advancing age, he still never lost his spark. This is from an August 28 interview.

“We have some comrades in our Party who have been too timid and who have patterned after the capitalist politicians whom I utterly detest. These comrades have no convictions about anything and are willing
to say or omit almost anything for the purpose of corralling votes. To them I want to say that if we get votes we are not entitled to, the crisis is coming when we have got to stand by our principles with our lives and if we invite a lot of people who are not ready to stand and fight, when the crisis comes, it will have been a calamity that we have had them at all. It will be the easiest thing in the world to get out in an average community and paint to them such rosy ideals that they will all want it, and then the first thing that goes wrong they will all turn and leave us. There we have a
peril, soft bricks with which we cannot build securely. Our object is not to build too rapidly, but to build permanently and for all time.”

On September 25 Debs had this to say. When I read it, I think it could have been written yesterday.

Exploitation, that’s what I mean; the conventional term for robbery. The exploitation of man by man, of nation by nation, enriching fabulously the favored few and impoverishing and degrading the toiling and producing masses.
The idle few have the power to rob and enslave the industrial many. Millionaires and tramps are alike the product of exploitation.
Less than 2 percent of our people privately own our natural resources and our industrial machinery.
In virtue of which they have the power to rule the nation and rob the people. That is the stupendous fact for the American people to realize, but Cox and Harding never hint at it, because they are the candidates of, and get their campaign funds from, that exploiting 2 percent.
As long as the industrial machinery that feed and clothes and shelters the people is the private property of the 2 percent minority of exploiting
capitalists, the people will be poor, life will be wretched struggle for existence, the divine in human nature will never be realized, and this world
will still be nearer to the jungles than to any real civilization.

The federal government refused to let Debs coordinate with his party leadership until very late in the campaign.

On December 25, 1921, President Harding commuted Debs' sentence to time served. In 1924, Debs was nominated for the Nobel Peace Prize.