Literally, sectionalism is a preference of the political class or those in various echelons of power towards a particular region, religion, race or certain people. It is a tendency of focusing more on the interests of a certain group and not all citizens as a collective whole. The exaggerated or more than necessary devotion, interest and deeds favoring a particular state, city or town, religion or culture, ethnicity or just people hailing from a certain class, background or parentage is sectionalism.
There are federal laws and state laws. The laws of one state may vary from the laws of another state but the federal laws stand applicable as they are, equally for all states. There cannot be one federal law for a few states and another for other states. If that happens, then it would constitute to be sectionalism. The federal government cannot say that a certain financial crime in New York City is acceptable or not punishable with a jail term and the same crime committed in San Jose will be treated with a jail term. The law must apply equally for any and sundry.
Sectionalism stems from the personal preference of politicians and socioeconomic compulsions. It can also stem from political and religious considerations. Throughout history, politicians have tried to appease people. They try to cater to their own people, which traditionally has been people of the same race, in the same region, having born and brought up in the same place, having a certain social and cultural upbringing, hailing from a certain financial background and having specific interests or preferences that match those of the politician.
Appeasement almost always leads to sectionalism. Also, unequal laws or any practice that is not made a standard will create turmoil in some sections of the society which will also lead to sectionalism.
Sectionalism in History
There are many historians who believe that one of the causes of the civil war was sectionalism. Some states were willing to abolish slavery, accept new trade policies and rights. Some states were not willing to accept them. This led to a conflict which snowballed into the civil war. Of course there were other reasons but there are credible historians and authors who stress that sectionalism was one of the major causes of civil war. Across the world, sectionalism is practiced in many ways. Appeasing immigrants and treating them very stringently or almost racially are both variants of sectionalism.