Senator Obama of Illinois rolled out a health care plan yesterday that he said would drive down spiraling insurance costs and offer coverage to the 47 million Americans who lack it.

“We can do this,” Mr. Obama told a crowd at the University of Iowa’s medical school in Iowa City, according to the Associated Press. “The climate is far different than it was the last time we tried this in the early ’90s.”

Mr. Obama’s proposal consists mostly of incremental changes in the current system under which most Americans get health insurance through their employer. He would require most employers to offer subsidized insurance or pay a percentage of their payroll to the government. Individuals and families would be eligible, regardless of pre-existing conditions, to buy policies currently offered to federal employees.

Mr. Obama’s prescription for American health care came under immediate, if indirect, criticism from two of his rivals for the Democratic presidential nomination, including a leader of the last major attempt to revamp the system, Senator Clinton.

“We commend Senator Obama for entering the health care debate,” a policy adviser to Mrs. Clinton, Neera Tanden, said, jabbing the Illinois senator for taking his time to craft a detailed proposal. “Senator Clinton believes that in addition to making health care more accessible, we have to achieve true universal health care so that every American has health coverage.”

The campaign of a former senator from North Carolina, John Edwards, was also dismissive of Mr. Obama’s proposals. “Any plan that does not cover all Americans is simply inadequate,” a spokesman for Mr. Edwards, Mark Kornblau, said.

Mr. Obama’s plan is less aggressive than one put forward by Mr. Edwards. To some extent on the health care issue, Mr. Obama now stands to the right of a former Massachusetts governor seeking the Republican nomination, Mitt Romney.

“It’s less coercive than the Romney plan,” a professor at Harvard Medical School, David Himmelstein, said. The plan signed by Mr. Romney in Massachusetts includes a requirement that all citizens who can afford health insurance purchase it. Mr. Obama’s proposal lacks this so-called individual mandate, which is also favored by Mr. Edwards and a former House speaker who may enter the presidential race on the Republican side, Newt Gingrich.

Mr. Obama’s plan also lacks a feature Mr. Edwards proposed allowing Americans to buy insurance directly from the federal government. “I don’t know why they didn’t put that in,” a professor of economics at Princeton University, Uwe Reinhardt, said. He said Mr. Obama’s announcement was short on specifics and probably involved too little federal spending to achieve full coverage.

“The Obama plan is much more of a sketch of an idea,” Mr. Reinhardt said. “Senator Edwards put money on it. Something like $120 billion a year, which is much more courageous. With $120 billion, you can really get to universal coverage.”

Mr. Obama’s campaign said his plan would cost between $50 billion and $65 billion a year and would be funded by rolling back President Bush’s income tax cuts for wealthy Americans.

Many aspects of Mr. Obama’s plan were borrowed from proposals put forward by other Democrats in recent years. The Illinois senator embraced a suggestion from the Democratic nominee in 2004, Senator Kerry of Massachusetts, that the federal government help pay for the costliest hospital stays and treatment.

Regardless of its details, the plan could sour some people on Mr. Obama, who has until now offered a vague policy agenda. “You cannot run an election indefinitely being all things to all people. Sooner or later you have to take stands,” a professor of political science at Hunter College, Andrew Polsky, said. “There’s a risk involved.”

For her part, Mrs. Clinton has also been slow to offer a detailed plan to achieve the goal of universal health coverage. “At this point, she doesn’t really need to endorse a plan. Nobody doubts that she’s very well informed on the subject,” Mr. Polsky said. “She has time.”

Mr. Obama said he could reduce current health care premiums by up to $2,500 a family each year by improving treatment of chronic illnesses and by encouraging hospitals and insurers to adopt modern information technology and electronic medical records.

Dr. Himmelstein said chronic illnesses need to be better managed but “there’s not an iota of evidence” that doing so saves money. The Harvard professor, who is a leading advocate for a single-payer system, expressed disappointment that no top-tier candidate has embraced that model. “Maybe Al Gore will get in,” the doctor said.