Sectionalism can best be defined as

BEFORE WE START:
Sectionalism has quite a few definitions. I don't know what it means, but I'm willing to do some research and provide you with the best definition.

DEFINE SECTIONALISM:
→When you are loyal to someone's region or their section of a country (if they're not "over" an entire country). 
→"Undue concern with local interests or petty distinctions at the expense of general well-being" -Google

WHY TWO DEFINITIONS?
This word could mean a few different things depending on how you use it, so I'm going to leave it up to you to find out which definition you want to use.

the loyalty to one's own region or part of the country, rather than to the country as a whole.

Say, being passionate about your home state or even hometown rather than being passionate about the USA.