Although Guiteau claimed that God was managing his case, his defense lawyer was his brother-in-law, George Scoville, an attorney whose 30-years’ experience included only two criminal cases.  At Scoville’s request, the court appointed a legal assistant, Leigh Robinson.  The prosecution team was headed by District Attorney George Corkhill and aided by John Porter, one of the most well respected lawyers in the nation, and Walter Davidge, another experienced criminal attorney.

After consulting with other attorneys, Scoville decided to drop the defense that the D.C. court lacked jurisdiction, and he also decided not to pursue the medical malpractice aspect, even though co-counsel Robinson thought it was the best defense.  Guiteau’s claim was that he had been temporarily insane for a month leading up to the shooting.  The first successful use of the temporary insanity plea was the 1859 case of Congressman Daniel Sickles, who fatally shot his wife’s lover.  However, the insanity defense in general was controversial and bitterly debated in the legal, medical, and public arenas, where it was often scoffed at as the “insanity dodge.”  Scoville wanted to prove that there was hereditary insanity in the Guiteau family.  The lawyer pointed to the fact that an uncle of the accused had died in an insane asylum in 1829, and he questioned the mental stability of a female cousin.  Guiteau’s brother, John, was angered by such an approach and unsuccessfully tried to dissuade Scoville and, when that failed, to replace him with another lawyer.

The prosecution hired Dr. John Gray, head of the mental asylum in Utica, New York, and editor of the American Journal of Insanity, who theorized that if a person who lived a moral life committed a heinous crime, the possibility of insanity existed; otherwise, the crime was the natural outcome of leading an immoral life.  In a pretrial interview conducted by Dr. Gray, Guiteau admitted that he was not insane under any medical definition of the word, but was in a legal sense because he had committed “an act without malice.”  During the trial, Gray sat at the prosecutors’ table, where he advised them on questions of the witnesses.

The trial commenced on November 14, 1881.  It was a major story in the national press, and the courtroom was packed to overflowing with journalists and curious onlookers.  Because competition for space was so keen, people brought picnic baskets rather than take a lunch break outside the courtroom.  When Judge Walter Cox refused to allow Guiteau to read an opening statement, the accused handed it to the press.  The document emphasized medical malpractice, not the gunshots, as the cause of Garfield’s death, and appealed for competent legal counsel.  Guiteau also wrote to President Arthur—“My Supposed Friend”—to seek his intervention “to tell the Prosecuting Attorneys to go very slow.”  Arthur did not respond to Guiteau, but replied later to a formal set of questions from the defense attorney.  Throughout the trial, Guiteau continually interrupted his lawyers, the prosecutors, and the judge.  On November 19, a drunken man on horseback shot at Guiteau, who was being escorted back to jail, piercing his coat but missing the prisoner.

On November 28, Guiteau took the stand, testifying for almost a week about the details of his life from childhood to his decision to assassinate President Garfield.  Prosecutor John Porter then began a rigorous cross-examination of Guiteau, with the intention of demonstrating that the accused was immoral and, therefore, not insane.  The defendant admitted to sexual relations with several women, and to contracting venereal disease.  While on the stand, Guiteau mimicked and mocked the prosecuting attorney, and interrupted to appeal for money to finance new counsel.  

One of the medical experts for the defense, Dr. James Kiernan, the managing editor of the Chicago Medical Review, weakened his case when the prosecutor informed the jury that Kiernan’s claim that 20 percent of the population was insane meant that at least two of them were bound for the asylum.  When the defense’s leading medical witness, Charles Spitzka of Columbia Veterinary College, was pointedly asked whether he was a veterinarian, he sarcastically responded, “In the sense that I treat asses who ask me stupid questions, I am.”