How did the composition of the House and the Senate shape the antebellum slavery debate?

The Representatives and Senators of the 36th Congress, which met from March 4, 1859–March 4, 1861, were chosen in the last national election held before the Civil War began in April 1861. However, two-thirds of the Senators had been elected in 1854 and 1856, and some of those whom the voters chose in 1858 were re-elected. Especially significant in the 1850s elections were the new states that were the former territories in the West and Southwest. The Senate had a Democratic majority, and the House had a Republican plurality. The Senators and Representatives from the Southern states were committed to making slavery an issue that the states—rather than the federal government—decided. In the Northern states, however, the preference for admission to the Union was predicated on rejection of slavery.
While more territories became part of the United States, and thus potentially entered the pipeline for statehood, few were actually admitted: California became a state in 1850, Minnesota in 1858, and Oregon in 1859.This meant that there were six more senators and a number of representatives in proportion to population. Kansas was especially crucial: In January 1861, Kansas became a state; it was the last state that would be admitted before the Civil War. The fight over slavery in Kansas was so hotly contested that the state became a battleground, known as “bleeding Kansas.” The future conflict over slavery was also predicted for the new territories organized in 1861: Colorado, Nevada, and Dakota.
As the House of Representatives grew proportionately to the population of the new states and the Senate gained two men per state, the adversarial lines became clear. The published history by the U. S. House states, “Slavery was never more politically divisive than during the 36th Congress….” The fight over slavery was in the front of the public’s mind in part because of John Brown’s 1859 raid. States’s rights, largely a coded term for support for slavery, was the most debated issue.
The 1860 national election which made Abraham Lincoln president sparked the move toward secession by numerous states. Thus, the 37th Congress had a very different composition, as the 11 states that seceded in 1861 were all Democratic; this left Republicans in control of Congress for the first time ever, as the Republican Party was newly constituted in 1860.