Here, Marty makes the case that when tensions are running high, especially due to racial segregation, politicians have an opportunity to figure out how to harness that energy. Those politicians, like Harold Washington, may have a bunch of very angry or emotional people willing to get out and fight for something better—if the politician in question can get them to mobilize. Barack chooses to go to Chicago when he realizes that he has no ties to New York—in that light, Chicago seems as good a place as any.