Who?
First in war. First in peace. Last in the American League.
Not the US, but an example of tribal rivalries used to be the case in Montreal in the NHL. Les Canadiennes were the French team. The Maroons were the English team.
Well if we’re extending to other countries, the three big soccer clubs in Calcutta are Mohan Bagan, East Bengal, and Mohammedan Sporting, which break along social lines – West Bengalis (Ghotis), East Bengalis (Bangals), and Muslims.
I am pretty sure Donnerwetter wasn’t talking about whole leagues but about the history of individual teams. As I understand it, e.g. the Dallas Cowboys never were anyone’s neighborhood amateur team with people coming to see the local boys play and to talk shit about their uptown rivals from two miles away.
No, the NFL was already a thriving, established league by the time the Dallas Cowboys came along, in 1960.
But the original teams of the NFL, which was founded in 1920, evolved out of Midwestern industrial leagues. In those leagues, teams were made up of workers from the same factories who played in their spare team. There USED to be a lot of industrial baseball AND football leagues. Originally, team names like “Steelers” and “Packers” were actual descritpions of the players.
Rather than within the same sport, the socioeconomic classes in the US have traditionally divided themselves by the sports they follow, though even here not to the same extent you might see elsewhere in the world and the distinctions growing less by the decade.
NASCAR vs Indy car, for example. Golf is more mainstream now, but it still carries a whiff of WASP-ishness. Tennis is sort of the same way. There are still honest to goodness polo enthusiasts (including a wealthy Houston drunk driving idiot who’s blown millions on developing the sport). Soccer is stereotypically primarily the domain of suburbanites and recent immigrants, particularly from Latin America. Basketball has a strong minority urban core. Etc. Etc. Etc.
American football is a weird exception. It’s considered more of a blue collar sport. But even here its development through universities means it’s always had a toehold across classes. The college and high school games have been some of the biggest drivers of football change, even in the professional leagues.
The funny thing is that probably 85% of the students at both schools are middle class/upper middle class kids whose graduating classes split evenly between the two schools- they were for all intents and purposes, the same.
For the Packers, it still is! 
In Wait Till Next Year, her memoir of growing up in the '50s, (Pulitzer Prize-winning historian) Doris Kearns Goodwin describes the tribal affiliations of Giant, Dodger, and Yankee fans, including the migration of the fans to the suburbs and of two of the teams to the West Coast.
And disaffected Dodger and Giant fans becoming Mets fans.
Check out some of the classic team names from the inaugural season of the American Professional Football Association, which became the NFL. There were 14 teams in the league, but they scheduled all manner of teams outside of the league.
It was the Canton Bulldogs signing superjock Jim Thorpe that really spurred public interest in pro football.