“It’s the basis of my longest standing friendships in this country,” Graham Randall, an English expatriate member of the Crew Union supporters group, said of his relationship with the team. “Now I see it through the eyes of my 11-year-old son. The news that the team might not be here at some point made me realize I would lose a huge thread in my life in the last 13 years.”

Since the missteps of the league’s early days, when M.L.S. acquired a reputation for gimmickry that it struggled to shake, the emergence of a genuine independent supporters culture has been viewed as a necessary step toward credibility, and it remains vital to the league’s vision of itself. But managing the dance between “organic” support and a strong league office can be delicate.

“The supporters groups are really smart and creative — I mean, you see the work that goes into the giant tifo displays alone — and they have smart leaders who communicate really well with the clubs and basically police themselves,” said Mark Abbott, the league’s deputy commissioner. “We don’t have to do much, though we do set down guidelines for acceptable behavior.”

Yet that desire for authenticity and passion often yields only symbolic displays of these qualities. It is one thing for fans to emulate a banner style imported from Italy, or the capo-led nonstop singing of South America, or the pop-parody chants of English soccer. But any organized fan groups that might actually feel and act on any of the enmity toward their rivals swiftly comes up against the realities of how the American sporting experience is managed and policed.

It also can make for absurdities on both sides, with teams who cultivate an “ultras” culture punishing anyone who runs afoul of an explicit code of conduct published on the league’s website. (“The use of streamers and confetti as an expression of fan enthusiasm is not prohibited,” one section of the code reads, “but will be kept under review.”)