Washington ? How did it get this bad on Capitol Hill?

Why does Congress barely function today?

The legislative branch of the world’s most powerful nation is now widely scorned as it lurches from one near-catastrophe to the next, even on supposedly routine matters such as setting an annual budget and keeping government offices open.

Congress is accustomed to fierce debate, of course. But veteran lawmakers and scholars use words such as “unprecedented” to describe the current level of dysfunction and paralysis. The latest Gallup poll found a record-high lack of faith in Congress.

There’s no single culprit, it seems. Rather, long-accumulating trends have reached a critical mass, in the way a light snowfall can trigger an avalanche because so many earlier snows have piled atop each other.

At the core of this gridlock is a steadily growing partisanship. Couple that with a rising distaste for compromise by avid voters. Unswerving conservatives and liberals dominate the two parties’ nominating processes, electing lawmakers who pledge never to stray from their ideologies.

Instead of a two-party system, American government has become a battle between warring tribes, says Mickey Edwards, a former Republican congressman from Oklahoma who has taught at several universities. When House and Senate leaders set out their goals and strategies, he said in an interview, “it comes down to the party first,” with the public’s welfare lagging behind.

The parties have driven all but a few centrists from their ranks. House districts are ever more sharply liberal or conservative because both parties collude in gerrymandering to protect incumbents and because mobile Americans like to live among like-minded people.

For many Republicans, the biggest threat to re-election is from their party’s right flank. For Democrats, the danger is being insufficiently liberal.

“The problem in a nutshell is that most members are more worried about their primary election than the general election,” said former Rep. Tom Davis, R-Va., now a campaign strategist. “They ask themselves, ‘Why should I go out and be the next Bob Bennett or Mike Castle?’ So they become very averse to compromise.”

Bennett, a three-term Utah senator, and Castle, a former Delaware congressman, were veteran GOP lawmakers who unexpectedly lost Senate nominations last year to tea party activists who had denounced them for occasionally working with Democrats.

Some Washington insiders thought the downgrade of the nation’s credit-worthiness, which followed last summer’s bitter battle over the government’s borrowing limit, might shock congressional leaders into ending their brinksmanship. But just days ago, a relatively minor disagreement over disaster aid money brought new threats of a government shutdown. Also, many lawmakers are deeply pessimistic that a special bipartisan committee can develop a viable plan this fall for sharply reducing the deficit.

Interviews with current and former lawmakers, congressional scholars and others point to several events that have tangled up Congress that lawmakers barely can keep the government’s lights on, let alone tackle big problems such as illegal immigration and soaring health costs. They include:

• Political realignment. Years ago, Southern conservative Democrats often worked with GOP lawmakers, and “Rockefeller Republicans” joined forces with moderate and liberal Democrats. Now, except for black enclaves, the South is overwhelmingly Republican. Liberal Republicans hardly exist, and even “moderate” Republicans face intense criticism from tea partyers and others.

• The 1994 Republican revolution. The GOP ended four decades of House minority status when Newt Gingrich of Georgia led an insurgency that would change Congress’ way of doing business.

“He greatly increased the party-versus-party polarization,” Edwards said. Republicans saw their mission as “less to be a lawmaker than to be a champion of the Republican cause, constantly at war, defeating Democrats.”

• Money’s role in polarization. New laws and tactics have steered millions of campaign dollars to interest groups on the far left and far right, and they spend it to defeat candidates they oppose.

“Most of the money is now not with the parties,” said Davis, who once headed the GOP House campaign effort. “There’s no centering force in politics,” he said. “The money has moved to the extremes. Everything has moved to the extremes.”