WASHINGTON (AP) — Stand by for one of the nastiest and, perhaps most economically dangerous U.S. political fights in recent memory.

As the languorous Washington summer draws to a close, Congress returns to the capital next month with stark battle lines drawn on spending issues that, if left unresolved, could shove the United States into defaulting on its debt for the first time or force the government to shut down for lack of funding. Conservative Republicans threaten one or both unless Democrats and the White House surrender to right wing demands to slash spending for President Barack Obama's health care overhaul.

While there is an internal debate in the Republican party over how to proceed, a growing number of legislators are concerned by what they perceive as overspending by the government, and they are determined to refuse action on the debt ceiling or the budget without major spending cuts from Democrats. Heated rhetoric stemming from the differences that divide the two parties — Republicans' desire to reduce spending on large benefit programs and Democrats' push for increased tax revenue — could cool as deadlines draw nearer. Republicans in particular are deeply divided on how far to push on denying a debt limit increase or shutting down the government over the budget.

The fight also concerns cuts that already have sliced huge chunks out of the defense spending, a burr under the saddle of Republicans, and government-funded social programs, part of the Democrats' political catechism. Congressional action is needed to lift those cuts, which were part of a 2011 deal that side-stepped debt default two years ago.

Those reductions run through 2021, but were intended to be so onerous to both political parties that they would be forced to compromise. That never happened as the nation's capital fell into the grip of a partisan stalemate not seen in decades.

Efforts to bridge the chasm during the August legislative break have proven a fool's errand, with neither side any closer to a big deal that would reduce the nation's deficit — the issue drives all others when it comes to any agreement on taxes and spending by an increasingly unpopular Washington government.

"The president has been trying for months, privately and quietly, with Republican leaders in the House and Senate to work out some sort of reasonable compromise. As of this date there is no evidence of progress," said Sen. Dick Durbin, a Democrat who is assistant majority leader and whip. "I'm afraid we're going to go to the brink. I hope I'm wrong."

The White House says it will not negotiate over health care reforms, period.

Here's what's at stake as soon as Congress returns on Sept. 9.

First, the federal budget year ends on Sept. 30, and there is no agreement between members of the Republican-controlled House of Representatives and the Democrat-controlled Senate on a budget for the fiscal year that begins Oct. 1. Without a deal on a new spending plan or at least an agreement to temporarily continue funding at current levels, the government would be forced to shut-down. That happened in the mid-1990s when Bill Clinton was president, and it cost opposition Republicans dearly.

Then, within a couple of weeks, the government will reach its limit on borrowing, known as the debt limit. Unless Congress agrees to raise that limit, the government would likely default on some of its debt, which would be a first in the country's history. Until Obama was elected president in 2008, raising the debt limit was a matter of course. But Republican threats to block the increase in 2010, when they had regained control of the House, caused one of the global rating institutions to lower U.S. government creditworthiness for the first time.

The primary Republican target this time is Obama's signature legislative achievement, the overhaul of the U.S. health care system. Republican legislators are threatening to impose a government shutdown over the budget or to nix a rise in the borrowing limit and force a debt default if Democrats and Obama do not knuckle under opposition calls to slash spending for the health care law. Much of that law begins taking effect later this year and early in 2014. House Republicans have already taken 40 votes to repeal or cut funding for the overhaul, moves that were never taken up in the Senate and would have been vetoed out of hand by Obama.

"We're going to have a whale of a fight," Rep. John Boehner, the speaker of the House, said at a fundraising appearance this month for a fellow Republican in Idaho.

The health care law forces all Americans to buy health insurance — offering government subsidies to low income citizens — and Republicans philosophically view that as wrongful government intrusion into private decision-making. That part of the law is what will help finance its regulations that will provide insurance to tens of millions of Americans who now are uninsured or who can't buy coverage because they already have a medical condition. Some Republicans are so opposed to such programs that they would like to see major cuts in the government's long-standing Social Security old-age pension system and the government-run Medicare health insurance program for Americans over age 65.

"I've made it clear that we're not going to increase the debt limit without cuts and reforms that are greater than the increase in the debt limit," Boehner said. "The president doesn't think this is fair, thinks I'm being difficult to deal with. But I'll say this: It may be unfair but what I'm trying to do here is to leverage the political process to produce more change than what it would produce if left to its own devices."

While more moderate Republicans in both legislative chambers are believed to oppose such drastic tactics, those in leadership positions such as Boehner are seen as captive to the growing power of the so-called tea party faction in their party, which is made up of legislators determined to cut taxes, shrink government spending and eliminate federal indebtedness.

That's because the most extreme elements in both parties hold outsized power in determining which candidates make their way onto the ballot. Candidates for Congress win a spot on the ballot in their states and districts through success in primary elections. Votes in those candidate-selecting elections are overwhelmingly cast by the most conservative Republicans, even though they might be a minority of the electorate. That reality forces even some of the most moderate Republican to adopt hard-line positions to fend off primary election challenges.

That leaves Democrats like Durbin hoping, perhaps against hope, for a change of heart among some of the opposition.

"The only hope I have is that the Republicans realize they tried this initially and the American people rejected it completely," he said. "Congress in general has a very low approval rating, but Republicans in Congress are even lower than Democrats."

To hear Boehner, that's not the case.

"I wish I could tell you it was going to be pretty and polite, and it would all be finished a month before we'd ever get to the debt ceiling. Sorry, it just doesn't work that way," he said on his swing through very conservative Idaho.

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EDITOR'S NOTE — Hurst is AP international political writer and has covered foreign affairs for more than 30 years.