At the heart of President Obama's drive to rein in health costs is a proposal for federal review and regulation of health insurance premiums, with a new agency empowered to block excessive rate increases.

State officials are leery of the proposal, which raises a host of questions: How would Congress define 'excessive'? How would the new federal power relate to state insurance regulation?

The proposal has great political appeal. But experts see a serious potential problem: Federal officials will focus on holding down premiums while state officials focus on the solvency of insurers, the ultimate consumer protection.

Economists say that holding down premiums does not necessarily hold down the cost of care, which reflects the prices charged by doctors and hospitals and the volume of services.

State officials worry that they would be left to police the solvency of health insurance companies while federal officials pressured insurers to reduce premiums, as Mr. Obama has done in recent days.

'You can't separate the underlying solvency of companies from the rates they charge,' said Sean Dilweg, the insurance commissioner in Wisconsin. 'The federal proposal would be a huge pre-emption of decisions that states have made over their history.'

Mary Beth Senkewicz, a deputy insurance commissioner in Florida, said, 'If you divorce rate-setting from financial oversight, that's a fundamental flaw.'

'Premiums must be reasonable in relation to the benefits,' Ms. Senkewicz said. 'That becomes a fairly complex analysis.' Insurance commissioners said they fully supported efforts to expand coverage and rein in health costs. But they said it would be risky to hold down premiums before costs were under control. And they do not expect the federal legislation to drive down costs anytime soon.

Sandy Praeger of Kansas, one of several insurance commissioners who met with Mr. Obama at the White House last week, said: 'From a consumer protection standpoint, the most important thing we do is ensure the solvency of companies. We would strenuously resist not having the ability to approve rates or having the commissioners' oversight of rates overturned.'

'You are not necessarily helping the consumer if you keep rates artificially low,' Ms. Praeger said. 'What's worse for the consumer: having a premium increase or having to pay the full amount of a medical expense because the company is out of business?'

Mr. Obama has cited his proposal for a Health Insurance Rate Authority as one of the most significant elements of his plan to remake the nation's health care system.

Representative Jan Schakowsky, Democrat of Illinois, said that in a meeting with liberal Democrats last week, Mr. Obama 'focused in particular on the new provision that would allow the Department of Health and Human Services to block exorbitant premium increases.'

As an example, Mr. Obama has pointed to a request by Anthem Blue Cross to increase premiums for individual policyholders in California by an average of 25 percent, with some rates going up as much as 39 percent.

Kathleen Sebelius, the secretary of health and human services, called Monday on five big insurers to provide detailed public justifications for their latest premium increases.

'If insurance companies are going to raise rates, the least they can do is tell us why,' said Ms. Sebelius, who has called their profits 'wildly excessive.'

The president's proposal is modeled on a bill introduced by Senator Dianne Feinstein, Democrat of California, which would give the health secretary the authority to deny or modify premium increases that she found unreasonable.

'We are the only industrialized nation that relies heavily on a for-profit medical insurance industry to provide basic health care,' Mrs. Feinstein said. 'I believe, fundamentally, that all medical insurance should be not-for-profit.'

In a recent report to Congress, the National Association of Insurance Commissioners said, 'Most states require insurers in the individual market to obtain prior approval of proposed rate increases before putting them into effect.'

At least 27 states have 'prior approval' requirements. In 12 other states, rates must be filed with regulators before their use, and regulators often have the authority to block increases. In other states, insurers must inform state officials of rate increases or file their rates along with standard contract forms.