Large gaps have also opened on the social safety net, minority preferences and immigration. Some centrists are alienated; more count themselves as "independents."
A crucial difference between interest-group and ideological politics is what motivates people to join. For interest-group politics, the reason is simple — self-interest. People enjoy directly the fruits of their political involvement.
Farmers get subsidies; Social Security recipients, checks.
By contrast, the foot soldiers of ideological causes don't usually enlist for tangible benefits for themselves but for a sense that they're making the world a better place. Their reward is feeling good about themselves.
I've called this "the politics of self-esteem"— and it profoundly alters politics. For starters, it suggests that you don't just disagree with your adversaries; you also look down on them as morally inferior.
It's harder to compromise when differences involve powerful moral convictions. Indeed, if politics' subconscious payoff is higher self-esteem, it makes sense not to cooperate at all. Consorting with the devil will make you feel worse, not better.
What's more satisfying is to prove your superiority by depicting your opponents as dangerous, thoughtless and morally bankrupt. Cable TV and the Internet feast on such outbursts.
All this relates to the present. Why do House Republicans persist when the self-inflicted damage is so great? In a new CBS poll, 44 percent blamed Republicans for the shutdown compared with 35 percent for the Democrats. One answer is that "standing on principle" bolsters their self-esteem.
Similarly, the Affordable Care Act ("Obamacare") — the cause of so much conflict — exemplifies the politics of self-esteem. Its main advocates, starting with the president, all have health insurance; they won't benefit directly.
But the ACA serves as a platform to assert their moral superiority. They care about people, while their opponents are heartless.
(Ignored is collateral damage: the corrosive effect on public confidence, for example. Also ignored is the fact that improvements in people's health will, at best, be modest. Many uninsured are healthy; others already receive care.)
The triumph of ideology is one of the great political upheavals of recent decades. It is, of course, partial; it coexists and always will with interest-group politics.
It's also full of paradoxes. On both the left and right, many activists are intelligent, sincere and hardworking.
But the addition of so many high-minded people — usually "true believers" in some cause — to the political system has made it work worse. It increasingly fails to conciliate or, on many major issues, to decide.
Samuelson is with The Washington Post Writers Group.