Free speech principles were often at stake in the antebellum controversy over slavery. In every case, proslavery advocates took the offensive in seeking to suppress the rights of their adversaries. Abolitionists attacked slavery as an institution, but they never seriously questioned the right to advocate on its behalf. Slaveholders, by contrast, fought to suppress free speech whenever they had a plausible chance of doing so. They fought to “gag” the reading of abolitionist petitions in Congress, and to prevent the postal system from circulating antislavery writings in the South.The mechanism for enforcing this ideological conformity did not, contra Jonathan Marks in Commentary, mainly come from the state, but rather the power of the mob (emboldening mine):
It is true, however, that the violent reaction of Southerners to any criticism of slavery did not entail a flat repudiation of free expression in principle. The history of the antebellum South shows how a society ostensibly protected by the first amendment can suppress dissent. While traveling in the antebellum South, the journalist and Irish immigrant E.L. Godkin explained why Southerners preferred to rely on mobs rather than laws:Moreover, the analogy of the social justice left taking the side of the south becomes even clearer once you factor in Lincoln's remarks from the debate at Alton, "You work and toil and earn bread, and I'll eat it." Demands for others' work products — medical care particularly, but also free tuition, the voiding of debt, etc., etc., etc. — are their stock in trade, as the early dialogue has proceeded. They do not oppose slavery, or even fractional slavery, so much as they object to its being racist.
The fact is, I imagine, that while every man in the country feels it to be necessary to the safety of the existing state of things to prohibit, absolutely and completely, all discussion as to the right of the masters to their slaves, no one likes to establish a censorship of the press by statutable enactment. This would be rather too close an imitation of absolutism. As long as it is only ‘the mob’ or ‘the public’ that maltreat a man for free speech, the credit of the state is saved…The emperor of Austria, Godkin continued, could only dream of angry mobs willing to do his dirty work for him, gratis. How that Emperor would have swooned at the glorious potential of Twitter!