The North and the South's fears of each other's influence on the course of national affairs:
The Industrialists of the North wanted tariffs. Tariffs artificially raise the price of imports so that domestic manufacturers can raise their prices too and thus make more money. It is legalized robbery. The people of the South did not like to pay artificially inflated prices for their goods.
The Industrialists and Merchants of the North wanted the government to finance improvements to harbors. The government has no money of its own; before it can give one party improved harbors, it must take the money from another party. The people of the South did not want to pay the cost of improvements to harbors.
The people of the South perceived that northern industrialists were bent on acquiring an empire for themselves in Mexico, the Carribean, Central America, and the South. To do this, Southerners believed, the northerners would first abolish slavery so as to bankrup the southern planters; then they could control the labor and land resources of the South and then move on to Central America and the Carribean.
The people of the North perceived that the U.S. Senators from slave states would continue to prevent bills for tariffs, harbor improvements, and railroads, if they continued to hold half of the seats in the Senate. So northerners started agitating to prevent slavery in the territories so there would be no new slave states, only new free labor states. The people of the South saw that not having a Senate evenly divided between the two predominant political/social/economic interests, but controled by the northern states, would make the southern states only subordinant, subject provinces of the North. Southern states would no longer be equal with northern states in their own country, the U.S.A.
The southern people were fearful of the turn that social affairs were taking in the North. Atheism and Unitarianism were becoming common in the North; free-love was being advocated and in some cases practiced; socialist communes were being established; divorce was becoming common. Some in the South said that the southern states should seceed from the Union so that these social sicknesses could not spread to the South.
In the North, there were some people who thought the South might gain control of the government and make slavery legal in all states. This was not very likely because there were a lot more people in the North than in the South, therefore the North would always have retained control of the House of Representatives. There probably were not many people in the North who thought this, but Abraham Lincoln thought there were some. He played on this fear in one of his political speeches and he would not have thought that he could make political capital on this fear if he did not know anyone who held this fear.
On your computer, search for University of Michigan DeBow's Review. Within DeBow's Review, search for Python. The first two articles on the resulting list are very good about southern perceptions of the North. They were published in 1857. I tried to establish these links below, but they would not stick. You may enjoy a short book titled The Kingdom of Matthias: A Story of Sex and Salvation in 19th-Century America by Paul E. Johnson & Sean Wilentz.
The main reason that they each feared each other's influence on national affairs is that each of the sections of the country had different needs. For example, a major issue arose over the Tariff of 1828. The South needed imports and exports while the North wanted its industries to be protected from foreign competition.
Other than that, I'm not sure what you're looking for. There were lots of incidents that made one side or the other uneasy. For example, the North disliked the idea of the Fugitive Slave Act of 1850 while the South didn't really like the fact that the Compromise of 1820 put the line so far south... Is that what you're asking?