Final answer:
The Free Soilers was the group that opposed the proslavery legislature in Kansas, setting up a rival government in Topeka and adopting a constitution that barred slavery.
Step-by-step explanation:
The group opposed to the proslavery legislature in Kansas was known as the Free Soilers. During a volatile period known as Bleeding Kansas, proslavery and antislavery factions clashed dramatically. The proslavery side, which included the border ruffians, was initially in control of the territorial government, with their power base in Lecompton, Kansas. In response, the Free Soilers, who were against the spread of slavery into the territories, set up a rival government in Topeka, Kansas, and adopted a constitution barring slavery.
Events such as the Sack of Lawrence and the Pottawatomie Massacre highlighted the tensions that could lead to violence between the opposing factions. These clashes between Free Soilers and proslavery forces contributed significantly to the rising national tension that would eventually lead to the American Civil War.