The Battle for Free Speech in Antebellum America

Free speech was under attack during the abolitionist movement. The south had effectively and legally blocked debate and free speech on the matter of slavery. It was illegal in all slave holding states to distribute literature of any sort which advocated any lessening of the system of slavery. Southern legislatures, not thinking that having their own laws on the books were enough, appealed to northern states to make it illegal to use the post office to distribute any such materials or to even publish them for distribution in the south.

Resolved, that the State of Virginia has a right to claim prompt and efficient legislation by her co-states to restrain... and to punish those of their citizens... who... assail her safety and tranquility, by forming associations for the abolition of slavery, or printing, publishing or circulating through the mail or otherwise, seditious and incendiary publications design, calculated or having a tendency to operate on her population.

Wendell Phillips speaking on behalf of abolitionist discoursed on the philosophy of the movement:

What is the denunciation with which we are charged? It is endeavoring, in our faltering human speech, to declare the enormity of the sin of making merchandise of men, of separating husband and wife, taking the infant from its mother, and selling the daughter to prostitution of a professedly Christian nation... the south is one great brothel, where half a million of women are flogged to prostitution, or, worse still, are degraded to believe it honorable.

Free blacks were a vital part of the abolitionist movement. And in time, as they became more vocal, there would be some divisions between black and white. first in philosophy, and then in a great divide where women's rights and feminism, began to become a greater issue of importance. This was seen by some as a distraction. Another fracture was based on the anti-political stance of Garrison. Wendell Phillips and others began to break off, throwing support to Lincoln for President, while Garrison eschewed politics. The result of this was in effect the forming two separate abolitionist movements, neither of which was as strong as the former.

Many free blacks resented the racism and paternalism of the whites an preferred to focus their efforts by holding all-black negro conventions. Frederick Douglass speaks of Garrison in effect wanting to gag his speeches for he seemed "too intelligent" and "unbelievable" they wanted him to stick to merely telling his story, which the White abolitionist would be the ones to editorialize and extemporize upon the moral and ethical issues of his tale.

Meanwhile, in Congress, the issue of slavery was barred from debate in the House of Representatives, thus effectively barring it's discussion in the upper house, due to the "gag rule" passed by southerners and Democrats, which was in effect from 1840-44. The gag rule stipulated as follows:

That upon presentation of any memorial or petition praying for the abolition of slavery or the slave trade in any District, Territory, or state of the Union an upon the representation of any resolution, or tother paper touching that subject, the reception of such memorial, petition, resolution or paper, shall be considered as objected to, and the question of it's reception shall be laid on the table, without debate, or further action thereon.
that no petition, memorial, resoution or other paper praying for the abolition of slaver in the district of Columbia, or any state or territory or the slave trade between the states or territories of the unied states in which it now exist, shall be receive y this house or entertained in any way whatever.

John Quincy Adams, former President and defender of the Amistad captives, was the main opposition to the gag rule. he succeeded in overturning it, after being denied the ability to bring a bill to the floor to proposed that beginning on July 4, 1842, every child born on American soil would be free, and that no slave trading could occur in the District of Columbia after July 4, 1845. William Slade of of Vermont and Joshua Giddings of Ohio were also Adams' supporters.

The Republican party itself was formed as an opposition to slavery, directly and indirectly.

from Wikipedia:

The Republican Party was first organized in 1854, growing out of the "anti-Nebraska" coalition of old Whigs, free soil Democrats etc. who mobilized in opposition to Stephen Douglas's January 1854 introduction of the Kansas-Nebraska Act into Congress, a bill which repealed the 1820 Missouri compromise prohibition on slavery north of latitude 36° 30' in the old Louisiana purchase territories, and so was viewed as an aggressive expansionist pro-slavery maneuver by many. Besides opposition to slavery, the new party put forward a progressive vision of modernizing the United States—emphasizing higher education, banking, railroads, industry and cities, while promising free homesteads to farmers. They vigorously argued that free-market labor was superior to slavery and the very foundation of civic virtue and true American values—this is the "Free Soil, Free Labor, Free Men" ideology

Charles Sumner was also key in the struggle. A Massachusetts Republican Senator challenged segregated schools in Boston. he hated racial discrimination and slavery, and fought to have slave owners compensated from the national treasury. After the 13th amendment, he went on to fight for full racial equality.