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."

When Democrats regained the majority for four years starting in 2007, they did not bring back the days of letting the minority party offer alternative bills. In fact, the House minority now plays a vastly diminished role, and both parties spend huge energies trying to gain or hold the majority.

• Cultural shifts. Unlike two and three decades ago, most lawmakers now keep their families back home, and many spend as little time in Washington as possible. They rarely socialize or talk politics across party lines, further discouraging compromise.

The media world has been reshaped by a decline in traditional, straight-news outlets and the rise of opinionated blogs, cable TV shows and talk radio. Republicans "live in mortal fear of Rush Limbaugh outing anyone" for being insufficiently conservative, said Rep. David Price, D-N.C., a former Duke University political science professor.

Rep. Jim Cooper, D-Tenn., in one of several essays on Congress' decline, writes that "Fox and MSNBC ... certainly inflamed partisanship." Social media, he says, has "popularized nonfact-based reality."

• Unrestrained use of partisan tools. Until the mid-1990s, the House majority often let the other party offer legislation for debate and votes. The measures typically failed, but the practice gave the minority a chance to air its philosophies and push for compromises where possible.

That rarely happens now. When Rep. Dennis Hastert, R-Ill., was speaker, he adopted a "majority of the majority" rule, which essentially made the minority party irrelevant. He would let no major bill pass without support from most of his fellow Republicans, even if it would pass easily with Democratic votes and just under half of the Republicans' backing.

Bigger changes occurred in the Senate. The powerful filibuster tool was used sparingly throughout most of the 20th century. But both parties now routinely employ it, enabling the minority to block almost any bill if its members stick together.

Unrestrained use of the filibuster contributes heavily to gridlock, Edwards said. "It's a failure of character in my view, of leadership for whichever party is in charge," he said.

• 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."

"The voters bear some blame," Davis added, noting recent elections in which the greatest energies were on the edges, not the middle.

Edwards, who has written extensively on government since leaving Congress in 1993, said in a recent article for The Atlantic that Americans have created a political system "that makes cooperation almost impossible and incivility nearly inevitable."

There's hope it might improve somewhat, over time, Edwards said in the interview. But for now, he said, members of Congress are responding to voters who say "if you compromise, if you reach across the aisle, we will defeat you."