Until recently, Seattleites had plenty of reasons to feel superior: A diet that includes mushrooms and berries foraged from pristine local forests, a commitment to fixed-gear bicycles, fair-trade coffee, facial hair and the best attendance of any city with a Major League Soccer team in the country by a landslide.

This season, Seattle Sounders fans aren't feeling quite so special anymore, thanks to a new MLS club with an eerily similar, comparably rabid and equally rain-impervious fan base that also hails from a progressive...