ANKENY, Iowa -- Amid an apparently tightening competition for the favor of Iowa Democrats, New York Sen. Hillary Rodham Clinton yesterday renewed her criticism of the health care proposal of one of her chief presidential rivals, Illinois Sen. Barack Obama.

Mrs. Clinton returned to the sharp critique she had emphasized at the Democratic candidates' most recent debate as she charged that the Obama plan, because it would not require individuals to obtain health insurance, would fall short of the goal of universal coverage while imposing a hidden tax on those who are insured.

"If we don't have universal health care, then we will be betraying the Democratic Party's principles," she said in an appearance at the Des Moines Area Community College campus in Ankeny, a suburb north of Des Moines.

Mrs. Clinton underscored her experience on the issue, as she greeted the audience with the observation: "It's such a pleasure to be back here at DMACC. I was here in 1993, talking about health care."

Mrs. Clinton has been on the offensive all week against Mr. Obama on the health care issue, hoping to dent the momentum suggested by several polls. Surveys have consistently shown a tight race between those two and former North Carolina Sen. John Edwards in Iowa as the state's Jan. 3 caucuses approach. One, the ABC-Washington Post poll, depicted Mr. Obama with a lead, although it was within the poll's margin for error.

"He has called his plan universal, then he called it 'virtually universal,' but it is not either," Mrs. Clinton contended. "When it comes to truth in labeling, his plan simply flunks the test."

Mr. Obama has argued that his rival, the national front-runner among Democrats, was willfully exaggerating the difference between their health proposals. He has contended that his plan, despite its lack of a requirement that individuals sign up for coverage, would effectively move the country toward universal coverage by making insurance affordable enough that most people would choose to purchase it. He estimates that the proposal would bring coverage to 98 percent of the population.

But Mrs. Clinton claimed that her rival's policy would leave as many as 15 million uninsured across the country, and 100,000 without coverage in Iowa alone. "That's more than half the population of Des Moines," she said. "That's a huge difference for those who get left out."

Mrs. Clinton bolstered the emotional power of her argument by inviting on stage with her two Iowa women burdened by costs and frustrations of dealing with the current medical insurance system. While their situations may strengthen arguments for the need for health care reform in general, neither woman provided an example of the differences between the Democratic health plans.

One woman, Pamela Brennan of Garner, is a diabetic who has been unable to obtain insurance because of her disease, and has been left with medical bills that she estimates at between $15,000 and $20,000 for treatment of its complications. The other, Ellen Duffy of Waterloo, is a nurse who lost coverage through her employer when breast cancer kept her from work.

At the core of the health proposals of all leading Democrats is a requirement that insurers would not be permitted to deny coverage because of pre-existing health conditions.

Earlier in the day, Mr. Obama attempted to rebut Mrs. Clinton's criticisms in a conference call with Iowa reporters.

"The main difference that she is focusing on right now is that I believe that the reason people don't have healthcare is because they can't afford it, not because they don't want it," he said, according to a transcript released by his campaign. "That's why my plan focuses -- more than any other plan -- on making healthcare affordable. And by doing so, I will cover every American.

"Senator Clinton is arguing that the only way to get every American covered is if you force every American to buy health care. And, unfortunately, she hasn't told anybody how she would enforce this mandate. So until she clarifies what exactly she intends to do to enforce this mandate -- Is she going to fine people? Is she going to take other steps to enforce it? -- this is more of a political point that she's trying to make than a real point."

Several independent health care experts have endorsed the concept of mandated coverage, which is also a feature of the plan offered by Mr. Edwards, who was first among the leading Democrats to weigh in on the issue during this campaign.

But the Obama-Clinton back-and-forth points to a caveat for discussion of any of the leading Democrats' health proposals. While they are dense with information and coverage concepts, none includes specific dollars-and-cents details on issues such as the levels of tax subsidies envisioned or, as Mr. Obama notes, the precise penalties that would come with failures to obtain coverage.

Mrs. Clinton, for example, would limit health insurance premiums to a certain, unspecified level of income. That may be an attractive concept, but without knowing what the percentage is, one that is difficult to evaluate fully. Similarly, Mr. Obama asserts that his plan would actually reduce the health care costs of most Americans, though by how much in specific cases is more conjecture than blueprint.

While Mrs. Clinton's appearance was delayed by plane trouble for more than two hours, she took numerous questions from her audience, displaying practiced familiarity with the signature issues in her detailed answers.