Like the "old times" in Dixie, Northern Democrats (AKA Doughface Democrats) are not forgotten. They formed a key part of the equation by which Southern or slaveholding interests held control all three branches of Federal government. You should be aware that the time frame is 1855, and the turn of events involved the debate in Congress over the Kansas-Nebraska Act. The number of seats in the House of Representatives should be 234.
McDonald [below] notes that Southerners were as confused by all this as you seem to be. "Southerners of all descriptions were mystified by the charge that Douglas’ bill was an act of southern aggression and the slavocracy ruled Washington."
Before going further, what I believe has no importance and is unrelated to the question. The question concerns what people in the North believed in the years immediately before the Civil War. Although there's more, such as low tarrifs being blamed for widespread unemployment in northern cities, here's the essence of what they believed.
Throughout the northern states, people were still rankled by the Fugitive Slave Act, and introduction of the Kansas-Nebraska bill deepened their resentment. More tellingly, the bill convinced moderate northerners that the slaveocracy, about which the abolitionists had been screaming, actually existed and was in full control of the government. During the spring of 1854, as the bill wended its way through Congress, mass meeting after mass meeting adopted resolutions condemning the measure, as did legislatures and city councils. For the first time, respectable community leaders – ministers and merchants and newspaper publishers – joined forces with the abolitionists they had previously shunned. That development was ominous, involving as it did the radicalization of people who had formerly been moderate or indifferent. One newspaper in downstate Illinois, late in the year, caught the essence of the change. “Before the repeal of the Missouri Compromise,” it editorialized, “in all contests between slaveholders and the abolitionists our sympathies were decidedly in favor of the former,” but this “act of treachery” altered everything.Douglas miscalculated the reaction in the South as badly as he had misjudged that in the North. He had supposed that the measure would win him favor in the South; many thought that he was seeking to win friends in the region who would later support his bid for the presidency. Instead, southern Democrats approved it, but not eagerly, and southern Whigs viewed it with attitudes ranging from lukewarm to hostile. Southerners of all descriptions were mystified by the charge that Douglas’ bill was an act of southern aggression and the slavocracy ruled Washington. Free states had a majority in the Senate, and despite the three-fifths clause, they had 144 of the 234 seats in the House. They cast 176 electoral votes to the South’s 120, and the incumbent in the White House was from New Hampshire.
And yet northern fears were not without foundation. Though California was a free state, it was dominated by southern Democrats, and its senators regularly voted with southern members. In the House, “doughface Democrats”, northern men with southern principles, ensured that no legislation unfavorable to the South would be enacted. Pierce’s cabinet included the fire-eater from Mississippi, Jefferson Davis, the secretary of war. Five of the nine Supreme Court justices were from the South. And writing Kansas off as unsuitable for slavery was uncertain. Cotton could not be grown in Kansas, but hemp, a slave-cultivated crop, could be, and the greatest concentration of slaveholding in Missouri was in the state’s northwestern corner adjoining Kansas.
If northerners needed further proof of the power and sinister designs of the slaveocracy, evidence was forthcoming in the official and unofficial manifestations of the Pierce administration's expansionist foreign policy. Three months after passage of the Kansas-Nebraska Act, Secretary of State William Marcy instructed the American ministers to Spain, France and Britain to meet in Ostend, Belgium, to formulate a policy for acquiring Cuba from Spain. The resulting Ostend Manifesto declared that obtaining Cuba was indispendable for the security of slavery, that the United States should attempt to buy the island, and that if Spain refused to sell, the United States should seize it by force. Nothing came of the project, nor did anything permanent result the next year when the adventurer William Walker sought to conquer Panama in a filibustering expedition. Both ventures, however, confirmed northern fears about southern intentions.