Recently, historians have argued that Southern politicians were increasingly uneasy that free-soil ideology might gain a foothold among non-slaveholding whites, who were expanding in numbers during this period. These anxieties, born of social causes, made southern leaders more aggressive and less willing to compromise. There certainly was growing inequality in the South among whites. In fact, the percentage of whites who owned slaves had been decreasing in the South ahead of the Civil War, even as the number of slaves in the region increased.
In 1857, there was a book, The Impending Crisis, published by Hinton Rowan Helper, a southern white man who was opposed to slavery for class reasons (as was typical of Free Soilers, he was not exactly racially enlightened). It terrified Southern slaveholders, who actually tried to have northern congressmen who had endorsed it barred from taking their seats. Here is a link to it: http://docsouth.unc.edu/nc/helper/helper.html
Generally, Northerners became less tolerant of slavery as an institution, though their reasons were primarily political, not social. The fugitive slave law and bloody border conflicts in Kansas contributed to this, and made Northerners less willing to compromise.