WASHINGTON - Sen. John Kerry has spent this week campaigning relentlessly on the problems in the nation's health care system and maintaining that President Bush has failed to address them.

The Bush campaign has countered furiously, saying Kerry's proposals are far too expensive and would inevitably lead to government micromanagement of private health care.

This is not just another exercise in partisan maneuvering. Nowhere are the policy differences between Kerry and Bush more apparent, health analysts say, than on what to do about rising health costs and the growing number of Americans without insurance.

Few dispute the extent of the problem. After several years of stability in the mid-1990s, the cost of coverage is soaring again, with premiums up an average of 13.9 percent last year, the third consecutive year of double-digit increases. More and more small businesses say they are staggering under the strain.

The number of Americans without insurance, meanwhile, has jumped to 43.6 million, according to a census report last fall, and more than a fifth are children.

Kerry said this week that these problems had worsened on Bush's watch.

"George Bush has had four years to offer America a real health care plan, and he hasn't," Kerry said Wednesday in Orlando, Fla.

Republicans say they are, in fact, responding: Senate Republicans on Tuesday endorsed a package of tax measures - including Bush's main proposal - aimed at the uninsured.

But the Bush and Kerry plans differ substantially in cost, the number of uninsured they hope to cover, the methods they would use and the underlying philosophy. Health care analysts say the difference in scale alone is striking.

Robert Reischauer, president of the Urban Institute and an expert on health, said, "The president's proposals remain very modest, while Sen. Kerry is willing to make health a major priority of his administration."

Stuart Butler, a vice president at the Heritage Foundation, put it differently: "Kerry is taking something of a risk looking at something on the scale he's proposing. Bush, on the other hand, is looking at the issue in terms that are within the scale Americans are able to handle right now."

Bush's main proposal for the uninsured would cost $70 billion over 10 years. It would give a new tax credit to low- and moderate-income families to help them buy health insurance. The proposal, first unveiled in the 2000 campaign but never enacted, would provide up to $1,000 for individuals and $3,000 for families. The administration estimates it would benefit 4.5 million Americans when fully implemented.

Bush presents the plan as a part of his philosophy of giving individuals more choices and more control over their health care, and of trusting the private market to respond to their needs.

"I've made my stand," Bush said in March. "I believe that the best health care policy is one that trusts and empowers consumers and one that understands the market."

Similarly, Bush proposes to hold down health costs through the approach sometimes described as consumer-driven health care. The idea is to make consumers more conscious of the cost of medical care, encouraging them to shop around for better deals and eventually reducing unnecessary services.

As the president envisions it, consumers would combine high-deductible insurance plans, which are relatively inexpensive, with tax-free health savings accounts that they would create to cover the cost of routine medical care. This year he has proposed making the premiums for those plans deductible, to further encourage their use.

In addition, Bush argues, Congress should make it easier for small businesses to band together to buy coverage.

Critics fault Bush's plan on several grounds. They say that his $3,000 tax credit falls far short of what it takes to buy a decent family plan, and that he relies too much on the market of individual health insurance, as opposed to buttressing the employer-based system of group coverage, considered far more stable. And many say those high-deductible plans would not lead to significant health savings and could discourage people from getting necessary care.

In the end, the critics assert, Bush's proposals would leave tens of millions of Americans uninsured, and many millions more squeezed by the soaring costs of an unfettered market.

Kerry, for his part, has a sweeping plan that tries to cover all uninsured children and most of the uninsured adults without the kind of fundamental structural change that doomed past Democratic proposals. It would cost $650 billion over 10 years, his campaign has estimated, and would be financed by rolling back the Bush tax cuts for those earning more than $200,000 a year.

The most unusual part of Kerry's plan would have the federal government pay 75 percent of the cost of the most expensive medical cases - those costing more than $50,000 a year - if employers guaranteed they would pass the savings along to their workers through reduction of premiums. This is aimed at easing the burden on businesses, especially small ones, and providing cost relief to Americans with insurance.

In general, Kerry would provide a variety of new subsidies to help small businesses and low-income people buy health insurance: $177 billion over 10 years in tax credits, more than twice the size of Bush's credits.

"He's spending a ton of money on trying to shore up the employer-sponsored system," Gail Wilensky, who ran Medicare and Medicaid in the first Bush administration, said of Kerry.

The senator would also create a new version of the Federal Employees Health Benefit Plan, a collection of private plans now available to Congress and federal workers, which would provide group coverage to other Americans and small businesses.

And he would expand assistance to the states to cover more children and low-income adults under Medicaid.

Kerry said this week that his plan would succeed because it is not a "government plan" with new mandates and bureaucracies. But Republicans say it would nonetheless lead to new government regulation and essentially transfer to the federal government the responsibility for a huge share of health care spending.