I agree, Democrats are just as tribal as Republicans these days, though Democrats are still more willing to compromise and Republicans are still more willing to play chicken with the global economy.

I think the Internet and cable TV have a lot to do with it, people have more choices, media have differentiated, so it is now profitable to chase a niche and difficult to chase the "centrist" masses.

But, as Democrats and Republicans have become more tribal, the number of people who identify with either is declining, so there is a growing independent middle, they just haven't formed a tribal identity of their own yet. If there were a more telegenic billionaire than Ross Perot who wanted to build a centrist tribe, maybe she'd have a chance.