For thirty years, Democrats pass multiple laws promoting and protecting slavery, culminating in 1850 with the Fugitive Slave Law. This law takes away all rights to jury trials, representation, and habeas corpus from any black who is so much as accused of being a slave.
In 1854, Democrats pass the Kansas-Nebraska act, opening up those territories to slavery, thus exceeding even the limits of the Missouri Compromise.
In 1854, the Republican party is formed to end slavery. Six of the nine planks in their fledgling platform statement deal with civil rights issues.