It's nearing lunchtime and the few people left on the hard chairs in the clinic waiting room are glancing at the television hanging high on the wall. In his examination room, Dr Jamal Gwathney has seen a two-month-old baby, a young woman with a heart pacemaker and chemical burns brought in after a fight with the police, and patients with asthma and diabetes.

But in clinics such as this, just across the Anacostia river from the great white dome of the Capitol, many of the ailments have an underlying cause: none of these people have access to adequate medical care.

"Their healthcare sometimes just takes a back seat to putting food on the table and a roof over their heads," says Dr Gwathney. "They don't come in until their health gets to a critical mass - until they have been having chest pain every day for two or three weeks, or until they start throwing up a little blood with that ulcer that they have and it gets them worried."

On the west side of the dome, in the K Street offices of the doctors profiled in local glossy magazines, medical practices are moving towards concierge-style care: for an annual fee topping $1,000 (£500), a trip to the doctor is akin to a visit to the spa, with appointments on demand, and even the most trivial ailments investigated by costly tests.

Between the two extremes is where America's healthcare system has unravelled. A patchwork of employer benefits and government assistance for the very poor and elderly has produced distinct differences. Those with very good jobs and generous benefits packages enjoy extensive, often almost wasteful, health cover. Meanwhile, tens of millions regularly put their health on hold because they cannot afford basic treatment, prescriptions, or even a visit to the doctor.

"We see a lot of things that could have been treated a lot earlier and even prevented, but they are unable to come in because they don't have health insurance and they don't have the funds to pay for it," says Dr Gwathney. He has spent three years at this free clinic, run by Unity healthcare in a poor, almost entirely African-American district of Washington. "You have to have a tough skin," he says.

The disparities seem to have brought America to a tipping point, and there is growing consensus - among business people, politicians, and economists - that the time has come for a change.

America spends more money on prevention and treatment of disease than ever before, yet it is falling behind on such basic indicators of health as infant mortality and life expectancy.

The US spends about 16% of GDP on healthcare, a proportion expected to climb to 20% by 2015, according to the National Coalition on Health Care. At present spending levels of $1.6 trillion a year, which works out at $6,700 per capita, is double what is spent in countries such as France. And yet that still leaves some 47 million Americans entirely without health coverage, and tens of millions of others under-insured, according to latest census figures.

It also fails to guarantee a better service to those Americans with access to healthcare. The US ranks last or near the bottom on quality, access, efficiency, equity and healthy lives, according to a report in May 2007 from the Commonwealth Fund, which studies healthcare.

"The US healthcare system is considered a dysfunctional mess," writes Ezekial Emanuel, chairman of the department of clinical bioethics, in a recent issue of the Journal of the American Medical Association.

Amy Robinson has known that all her life. She was born with a kidney condition that required 25 operations by the time she was eight. But in some ways the financial consequences have loomed almost as large as the disease.

By the time she was 21, and her kidneys were failing, she was spending up to $1,400 a month on medication as well as the dialysis paid for by the state. By 23, she had a kidney transplant. A year later, despite working two or three jobs to pay off her medical bills, she was forced into bankruptcy by debts of up to $10,000. Although the cost of the transplant and other medications were supposed to be met by insurance and state assistance, "things slipped through the cracks a lot".

Now 31, she works for a teaching union in Topeka, Kansas, and although she has insurance, she still spends up to $500 a month - nearly a third of her take-home pay - on medication, lab work, vitamins, and doctors' visits.

Hers is a common experience of the chronically ill in America, especially over recent years as the common expectation that a good job would automatically include health benefits has been repeatedly betrayed.

Since 2000, there has been a steady decline in the number of employers who offer health coverage, particularly among small businesses. Others are scaling back on the range of coverage. In part that is because providing health coverage has grown too expensive, with the lack of regulatory controls encouraging a steady rise in costs. The average cost of insurance premiums rose 7.7% last year, far above the rate of inflation or rise in salaries, says the Kaiser Family Foundation, which studies healthcare.

The rising costs have shifted the burden of cover on to the individual. In 1996, about 15.8% of adults under 65 spent 10% or more of their disposable income on insurance premiums and other healthcare costs. By 2003, it was 19.2%, says the Commonwealth Fund. For those at the lower end of the income scale, healthcare is not affordable.

"If you don't work for an employer who offers insurance your options are very limited unless you are a child or very, very poor," says Sara Collins, an economist at the Fund.

Healthcare experts say that there is sometimes no rational reason for the rising costs, and that there are huge disparities across the country. In Miami, for example, it will cost $11,352 a year to treat the average pensioner, but just $4,273 to treat one in Salem, Oregon, says the Dartmouth Atlas of Health Care. The cost of dying also varies from hospital to hospital, and state to state, the study found.

Americans say they are determined to fix a broken system. In March more than 30 major corporations from Safeway to Pepsi joined together to lobby for healthcare. America's unions have also adopted the cause and all three frontrunners in the Democratic race for the White House have tuned in early. One Republican contender, Mitt Romney, introduced a system of universal health cover as governor of Massachusetts.

None of the mainstream proposals would move America towards the national healthcare systems of Europe or Canada. That idea remains taboo. But there is strong support for building on the existing system to make sure all children have healthcare, and a growing proportion of the working poor.

Under the Massachusetts plan, which became law last year, all residents must buy private health insurance or face a fine. As well as extending coverage of children and the elderly, Massachusetts required employers to provide health insurance, or join an insurance scheme that would subsidise the cost for staff. Similar proposals are under consideration in states from Illinois to California.

On the campaign trail, meanwhile, John Edwards, who was the first Democratic contender to unveil his health plan, has proposed subsidising insurance for the middle class, a scheme he would fund by rolling back George Bush's tax cuts. His rival, Barack Obama, has proposed a similar fix, while Hillary Clinton has focused on cutting costs via better access to preventive medicine.

But if any of those initiatives are to cure America's health system, they will have to move quickly to help the likes of Amy Robinson. After paying for her medicine, she sinks $200 further into debt each month. "Every bit of money I spend is a decision. Even the McDonald's coffee I bought this morning. Every dime," she says. "I'm probably never going to get out of the hole. Even if the status quo holds with my kidney, all I am looking at is more medical bills."

Explainer: US and the NHS

Successive health secretaries, grappling with a vast and cumbersome NHS, have looked across the Atlantic for inspiration. But the most they manage is to cherry-pick an idea here and there. While they admire the skills and technological advances of the best US hospitals, no politician wants to risk electoral wrath by tampering with the founding principle of Nye Bevan's NHS - that healthcare provision in the UK should be universal and free at the point of access.

Even in the UK, though, some patients are more equal than others. Those in deprived areas and members of ethnic minorities are more likely to fall ill and receive poorer healthcare than their more affluent, argumentative compatriots. But the government recognises this inequality and tries to address it with funding.

However, the US has - with this government's encouragement - made inroads here. United Health, a US healthcare provider based in Minneapolis, arrived in the UK in 2004 to develop a scheme which had succeeded in keeping frail and elderly people out of hospital in the US, although an evaluation in November showed it had reduced neither the number of admissions nor deaths.

But arguably these developments in the UK are just adjustments. The National Institute for Health and Clinical Excellence decides which drugs the NHS can use, leading to patient outcries. But the government says a state-funded system cannot afford everything. Sarah Boseley