They call themselves a "nation" and gather in the fall or early winter, usually on Sundays. The faithful wear clothing emblazoned with the names of heroes; they pray by twirling a sacred talisman, a gold-and-black terry-cloth hand towel, at times achieving dervish-like ecstasy. I was among them on a recent Sunday at the Pour House in Washington, D.C., three blocks from the U.S. Capitol: hundreds of Pittsburgh Steelers fans, cheering at ear-splitting volume, in a crowd composed of myriad races, ethnicities and hometowns, many far beyond western Pennsylvania. They shared tales of past gatherings in, say, London (or Altoona), and vows of more, at future Super Bowls. "This fulfills a kind of tribal, religious appetite," said Matt Stelmack, 32, a political scientist who, as it happens, studies identity politics.

"Steeler Nation" is one of the planet's most populous and intense sports-fan cohorts. There are many others, of course, and have been for many decades. But such groupings—what might be called "voluntary tribes"—are assuming a new importance in America. As neighborhoods and schools become more diverse, marriages become more mixed and social hierarchies break down, old lines are getting blurry. Voluntary tribes are a way of recreating a sense of community.

More than "associations"—the kind Tocqueville noticed were so numerous in America—these are emotionally intense affinity groups based on shared aims, obsessions or political crusades, not on DNA. Fueled by the Internet, according to Princeton historian Julian E. Zelizer, they're "filling the gap of the neighborhood institutions of the 20th century." The ravages of breast cancer have created several such communities. The Avon Walk for Breast Cancer, featuring marchers of all ages, sizes and descriptions, has drawn 100,000 participants. The act of marching together (often, at first, as strangers) engenders friendships, caregiving circles and other charity work in widening gyres. "The march itself is a symbol of joining a tribe of concern," says Barbara Thompson, a child-development specialist in Washington who became a walk fundraiser.

In politics, the Obama campaign is the epitome of the voluntary tribe. Traditional parties once generated deep, tribelike emotion, but they tended to be assemblages based on race, ethnicity and region. Obama's tribe, by contrast, was founded on ideas—ending a war, changing the ways of Washington. "We cut across all the old categories in amazing ways," says Meredith Segal, who founded the first student site for Obama on Facebook in 2006. "An engineering major from MIT suddenly had a close tie, through Obama, to a fashion major from Texas." It won't be easy to maintain that fervor. In football, all you have to do is win the next game. In the presidency, you have to save the world.

© 2009