What is the significance of Bleeding Sumner?
It seems fairly unimaginable that a Congressman would attack a Senator on the floor of the U.S. Capitol building, leaving him injured for a period of three years. In 1856, this was the condition of the American political system. The Kansas-Nebraska Act allowed the citizens of the new territories to decide the issue of slavery. This law was meant to be a compromise between the North and South. It actually increased tensions as proponents of both sides of the argument descended upon Kansas to vote for, or against slavery. The tension led to violence throughout the territory and even tore apart the Whig political party.
In May of 1856, the tension of the slavery issue reached the Capitol building. Massachusettes Senator Charles Sumner had delivered an incendiary abolitionist speech to Congress two days earlier and southerners were looking for some retribution. On May 22nd, South Carolina representative Preston Brooks brutally beat Sumner with a walking cane. Sumner was knocked unconscious by the attack and it took him three years to recover and return to his job.
The attack further heightened tensions that already existed between the North and South. In the South, Brooks was lauded as a hero, but to Northerners, it demonstrated the violent and degrading behavior of the Southern mentality.
I assume that you are asking about Sen. Charles Sumner, who was a Republican from Massachusetts. He is the one who was attacked and badly beaten by Rep. Preston Brooks (a Democrat from South Carolina) after Sumner gave an anti-slavery speech in which he strongly criticized a Senator who was a relative of Brooks.
The significance of this event is that it is one of the things that helped to drive the North and South farther apart. Brooks came to be seen as a hero in the South, with people sending him canes to replace the one he broke beating Sumner. Northerners thought that the episode proved how brutal Southern slaveowners were. These reactions helped to push the two regions farther apart and helped to bring on the Civil War.