Why are so many people voting for the avowedly pro-slavery party?
The bold could easily apply to either of these parties. While in Massachusetts, anti-slavery candidates co-opted the Know-Nothing banner for the 1854 elections while the party system was in flux, this was the exception rather than the rule: the Know Nothings had their base of support among Southern Whigs and others unwilling to join the Republicans because they were explicitly in favor of slavery, and after Millard Fillmore (of Fugitive Slave Law fame) was nominated by the Native American National Convention in 1856, the anti-slavery minority quit the party. Representing the Know Nothings as anything less than a pro-slavery party with a nativist, anti-Papist twist is ahistorical.