On May 20, 1856, Senator Charles Sumner made a speech denouncing "The Crime Against Kansas" and the Southern leaders whom he regarded as complicit, including Senator Andrew Butler.[14] Sumner compared Butler with Don Quixote for embracing a prostitute (slavery) as his mistress, saying Butler "believes himself a chivalrous knight".

Of course he has chosen a mistress to whom he has made his vows, and who, though ugly to others, is always lovely to him; though polluted in the sight of the world, is chaste in his sight. I mean the harlot Slavery.[15]

Senator Stephen Douglas of Illinois, who was also a subject of criticism during the speech, suggested to a colleague while Sumner was orating that "this damn fool [Sumner] is going to get himself shot by some other damn fool."[16]

Sumner's language was intentionally inflammatory; Southerners often claimed that abolition would lead to intermarriage and miscegenation, arguing that abolitionists opposed slavery because they wanted to have sex with and marry black women.[17] Abolitionists reversed the argument by accusing southerners of loving slavery so they could have slave mistresses at their disposal. As Hoffer (2010) says, "It is also important to note the sexual imagery that recurred throughout the oration, which was neither accidental nor without precedent. Abolitionists routinely accused slaveholders of maintaining slavery so that they could engage in forcible sexual relations with their slaves."[18]

Brooks thought of challenging Sumner to a duel. He consulted with Representative Laurence M. Keitt (also a South Carolina Democrat) on dueling etiquette. Keitt said that dueling was for gentlemen of equal social standing. In his view, Sumner was no gentleman; no better than a drunkard, due to his supposedly coarse and insulting language toward Butler.[19][20] Brooks then decided to "punish" Sumner with a public beating.

On May 22, two days after Sumner's speech, Brooks entered the Senate chamber in company with Keitt. Also with him was Representative Henry A. Edmundson (Democrat-Virginia), a personal friend with his own history of legislative violence. (Edmundson had been arrested by the House Sergeant at Arms after attempting to attack Representative Lewis D. Campbell of Ohio during a tense debate on the House floor in May 1854).[21]

Brooks confronted Sumner, who was seated at his desk, writing letters. He said, "Mr. Sumner, I have read your speech twice over carefully. It is a libel on South Carolina, and Mr. Butler, who is a relative of mine." As Sumner began to stand up, Brooks hit Sumner over the head several times with his cane, made of thick gutta-percha with a gold head. Sumner was trapped under the heavy desk (which was bolted to the floor), but Brooks continued to strike Sumner until Sumner wrenched the desk from the floor in an attempt to escape.[22] By this time, Sumner was blinded by his own blood. He staggered up the aisle and collapsed unconscious.[23] Senator John J. Crittenden, Representative Ambrose Murray, and others attempted to restrain Brooks before he killed Sumner, but were blocked by Keitt, who brandished a pistol and shouted at the onlookers to leave Brooks and Sumner alone.[24][25][26] Brooks continued beating Sumner until the cane broke, then quietly left the chamber with Keitt and Edmundson.[27] Brooks required medical attention before leaving the Capitol, because he had hit himself above his right eye with one of his backswings.[28]

Sumner suffered head trauma that would cause him chronic pain and symptoms consistent with what would now be called traumatic brain injury and post-traumatic stress disorder, and spent three years convalescing before returning to his Senate seat. He suffered chronic pain and debilitation for the rest of his life.