I don't mention it because there was no parody in Congress between slave and free states. 1848 was the last time there was some even parody between free and slave states. Between 1848 and January 1861, California, Minnesota, Oregon, and Kansas were admitted to the Union. Texas had been the last slave state admitted in December of 1845, with Iowa and Wisconsin soon after in 1846 and 1848 respectfully. So the country saw six new free states admitted to the Union between 1845 and 1861.
The Senate was one thing, but the House was well in favor of the free States, only 90 of 241 representives came from slave states (that's all fifteen slave states that existed during the time of the 36th Congress).