Sens. Hillary Rodham Clinton and Barack Obama, knowing how worried Americans are about health care, are campaigning hard on promises to improve the system, whose spending is projected to reach nearly $2.4 trillion this year.

Both would make more affordable health insurance available to the uninsured, require many businesses to offer coverage or contribute to its cost, and ensure that people could obtain coverage even if they have pre-existing health problems -- all significant changes to the current system.

The key distinction between the two is that the Clinton plan would mandate coverage for all Americans. Mr. Obama would require coverage only for children, though he might consider a mandate for adults.

Analysts not associated with the campaigns emphasized that both plans build on the existing health care system. People who have health insurance that they like might see little change.

Dr. David Cutler, a Harvard University economist and health care spokesman for the Obama campaign, said he believed that insurance options in Mr. Obama's plan could be made so affordable and accessible that virtually everyone would voluntarily sign up.

Neera Tanden, the Clinton campaign's policy director, emphasized that a requirement to obtain coverage is critical to ensuring that most Americans enroll in a health plan.

Dr. Linda Blumberg, an economist and principal research associate at the Urban Institute, said studies suggest about 50 percent of eligible uninsured would sign up without a mandate.

Some people, particularly those young and healthy, simply aren't motivated to enroll and pay for coverage, she said. Even in public programs like Medicaid, where enrollment is offered to many recipients for free or at a nominal cost, some remain uninsured. But Mr. Obama's lack of a mandate for all, she acknowledged, has a certain political appeal.

"When you get mandates and requirements in place, Americans bristle," Dr. Blumberg said, noting a mandate for children might be accepted more readily.

"I think it's a political calculus," she said of Mr. Obama's decision not to call for a mandate for everyone, "but it has to come with the knowledge that you can't get as many people covered that way."

Another result of not having a mandate, she said, is that older, sicker people are more likely to enroll, increasing expected average spending in the new plan. And with a significant number of other people -- perhaps 15 million of the nation's 47 million uninsured -- not signing up for coverage, it would be harder to redirect funds now spent on the uninsured to help offset the cost of reforms, since safety net providers would still need a way to finance that care.

A mandate would be fairer in some respects because the system already is paying for patients who lack health insurance, said Dr. Sherry Glied, professor and chair of health policy and management at Columbia University's Mailman School of Public Health. A mandate also would make the insurance market work better and would decrease costs, she said.

Dr. Glied emphasized the similarity in the two candidates' plans and said prospects for either depend in part on the makeup of Congress.

A major source of funding for both plans would be a rollback of tax breaks for wealthier Americans. Both also contain provisions for efficiencies and redirecting funds.

The Clinton plan would cost an estimated $110 billion annually when fully phased in. The plan calls for no overall increase in health spending or taxes, though Mrs. Clinton has said she might consider increasing tobacco taxes if needed. Published reports suggest the Obama plan would cost up to $65 billion a year, though Dr. Cutler said the cost would be closer to that of the Clinton plan.

Assessing the exact cost is difficult, Dr. Blumberg said, because the campaigns have not disclosed certain details, such as the size of the subsidies that would be provided to individuals and businesses to make insurance affordable.

The plans also don't specify penalties that might result if people failed to obtain coverage for themselves or their children.

Both contain a long list of measures aimed at reducing the cost of health care, improving quality and expanding the availability of affordable coverage.

The Clinton plan would give Americans several options for health insurance. They could keep their current coverage or purchase other coverage through a new Health Choices Menu within the Federal Employee Health Benefit Program. The uninsured, other individuals and businesses could buy coverage through the menu, which also would offer a public plan option.

The Obama plan would create a National Health Insurance Exchange for individuals and businesses that want to purchase a private insurance plan or enroll in a new public plan.

Both candidates would provide income-related subsidies to help make insurance affordable. Spokespeople for the campaigns said at least partial subsidies might be provided for people with incomes up to 300 to 400 percent of the federal poverty level, about $52,800 to $70,400 for a three-member family.

The Clinton plan would make subsidies available through refundable tax credits. Mrs. Clinton has said she would favor health insurance premiums that would not exceed 10 percent of a family's income.

Both plans generally would require businesses to provide health insurance to their workers or contribute to the cost of coverage. The Clinton plan would offer tax incentives to small businesses to begin or to continue to offer health insurance. The Obama plan would reimburse employer health plans for a portion of their catastrophic costs if they guarantee the savings are used to reduce the costs of workers' premiums.

The Clinton plan also contains a time-limited provision to give tax credits to employers offering retiree health plans to offset a portion of their catastrophic health care spending as the Baby Boom generation ages.

The two plans would prohibit insurers from denying coverage based on applicants' pre-existing health problems; expand the Medicaid program and the children's health insurance program known as SCHIP; give flexibility to states or regions in offering coverage; allow the Medicare program to negotiate on prescription drug prices; and reduce payments to Medicare health maintenance organizations, which some have criticized as excessive.

To improve quality and cut costs, the plans call for investment in electronic health records, disease prevention strategies, improved management of chronic health problems and independent efforts to guide health care decision-making.

Analysts agreed the two Democratic plans are much different from health proposals advanced by Sen. John McCain, the presumptive Republican presidential nominee.

Mr. McCain's proposals would not guarantee coverage -- for example, for people with pre-existing health conditions.

He also has proposed eliminating tax breaks that individuals receive through their employer-sponsored health insurance. The new taxes on those plans would be offset by proposed tax credits of $2,500 per person or $5,000 per family. The tax credits also would be available to the uninsured.

Dr. Douglas Holtz-Eakin, an economist and senior policy adviser to Mr. McCain, said the change would "level the playing field" by eliminating the tax break for high-income people with generous health care plans. He said additional tax credits might be given to people with low incomes.

Dr. Blumberg expressed concerns that the tax break changes would undermine the group health insurance system by encouraging younger, healthier people to seek their own coverage, placing substantially increased financial burdens on older or less healthy people.

Dr. Holtz-Eakin said Mr. McCain's primary goal is to reduce the costs in the health care system through a variety of measures, such as improved preventive health care.