Sumner here undertook to prove that Congress exceeded its powers when it presumed to lay down rules for the rendition of fugitive slaves, and this task exceeded even his power as a constitutional lawyer.
The circumstances under which Sumner attacked slavery were such as to have alarmed a less self-centered man, for the two years following the introduction of the Nebraska bill were marked by the most acrimonious debate in the history of Congress, and by physical encounters, challenges, and threats of violence.

But though Congressmen carried concealed weapons, Sumner went his way unarmed and apparently in complete unconcern as to any personal danger, though it is known that he was fully aware that in the faithful performance of what he deemed to be his duty he was incurring the risk of assassination.
The pro-slavery party manifested on all occasions a disposition to make the most of the weak point in Sumner's constitutional argument against the Fugitive Slave Law.

He was accused of taking an oath to support the Constitution though at the same time intending to violate one of its provisions.

In a discussion, in June, 1854, over a petition praying for the repeal of the Fugitive Slave Act, Senator Butler of South Carolina put the question directly to Senator Sumner whether he would himself unite with others in returning a fugitive to his master.

Sumner's quick reply was, "Is thy servant a dog that he should do this thing ?" Enraged Southerners followed this remark with a most bitter onslaught upon Sumner which lasted for two days.