Northerners viewed the violence against Senator Sumner as an attack on "___."

Northerners viewed the violence against Senator Sumner as an attack on free speech.

Explanation:

The violence against Senator Charles Summer took place on May 22, 1856, during a disagreement between him and another senator regarding the maintenance of the slave system in the southern states of the USA. For Charles Summer, who was an abolitionist Republican, slavery was unacceptable in the United States and should be banned immediately. This raised the ire of southern politicians in favor of slavery, principally Deputy Preston Brooks, who was a Democrat in favor of slavery and in a moment of wrath, took a cane and beat Senator Summer.

The northern population was disgusted by this situation and saw this case as an illegal act of attack on freedom of expression. This helped to strengthen the enmity between northerners and southerners.

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