WASHINGTON — The government "shutdown" can teach us a lot about American politics. Even after it's over, a central question will remain. Why did they do it? Why did congressional Republicans trigger a shutdown for which they would predictably be blamed and from which they could win few Democratic concessions? The conventional answer blames stupidity, craziness and fanaticism. This is too glib and partisan. It misses a deeper cause that, I believe, helps explain why politics has become more dysfunctional.

By dysfunctional, I mean that it's less able to mediate differences and conflicts. This is, after all, a central purpose of politics. Broadly speaking, conflicts originate from interest groups and ideologies. The curse of U.S. politics is that it's become less about interests and more about ideologies — and ideologies breed moral absolutes, rigid agendas and strong emotions.

To be sure, interest-group politics can involve huge stakes and fierce disputes: farmers seeking subsidies, multinational companies plugging tax breaks, Social Security recipients protecting benefits. Sometimes compromises may satisfy everyone, but even when they don't, spillovers are usually modest. Although adversaries may detest each other, their ill will tends to focus on narrow disagreements.

By contrast, ideological differences are expansive and explosive. To me, "ideology" extends beyond an explicit political philosophy. It includes broad differences in lifestyles and basic assumptions about America's best interests. In recent decades, ideological issues have occupied more of the political stage for both left and right. The "size of government" is a proxy for a deep divide. The left sees bigger government as a tool for social justice; the right fears it's a threat to freedom. Moral crusades abound. "Saving the planet" (global warming) is one. Preventing "murder" is another; that's gun control for the left and outlawing abortion for the right. The left's campaign for "gay rights" is the right's quest to save the "traditional family."

Just why partisan differences have widened is controversial, but they clearly have. "Basic beliefs are more polarized along partisan lines than at any point in the past 25 years," said a 2012 Pew opinion study. One question asked about the need for "stricter laws and regulations to protect the environment." Among Democrats, 93 percent agreed, the same as in 1992; for Republicans, agreement was 47 percent, down from 86 percent in 1992. 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.

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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.

Robert J. Samuelson is a Washington Post columnist.

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