MADISON, Wisconsin — On an oppressively hot August day in downtown Madison, the signs of this famously liberal city’s progressive activism are everywhere. Buildings are draped in pride flags and Black Lives Matter signs are prominently displayed on storefronts. A musty bookstore advertises revolutionary titles and newspaper clippings of rallies against Donald Trump. A fancy restaurant features a graphic of a raised Black fist in its window, with chalk outside on the sidewalk reading “solidarity forever.”Yet the Green Party, which bills itself as an independent political party that has the best interests of self-described leftists at heart, is nowhere to...