PALO ALTO, Calif. — The issue of requiring individuals to purchase health insurance is triggering an escalating fight between the top contenders for the Democratic presidential nomination even as the same question is bogging down Governor Schwarzenegger’s plan to make sure all Californians are insured.

“It’s all connected and the connection here is a push for an individual mandate,” a critic of Mr. Schwarzenegger’s proposal, Carmen Balber of the Foundation for Taxpayer and Consumer Rights, said.

In the presidential race, Senator Clinton’s campaign and a variety of prominent commentators have gone after Senator Obama of Illinois for not including a so-called individual mandate in his plan to overhaul health care coverage. However, in California, some labor unions and liberal groups are attacking Mr. Schwarzenegger precisely because his plan would force individuals to purchase insurance, while making no promises about what that insurance would cost.

“If you’re not limiting what private insurers can charge, then that is, in the end, an unlimited burden on individuals,” Ms. Balber said. She said such a mandate, without regulation of premiums, could also be a bonanza for private health insurance providers.

“It’s a great illustration of the power of the insurance industry dime in politics,” said Ms. Balber, whose group claims to defend consumers against “Big Business” and has employees with ties to Ralph Nader’s consumer movement. “If you give them a guaranteed customer base and say, ‘Charge them whatever you want,’ it’s a bonus for them.”

A policy analyst for the California Labor Federation, Emily Clayton, said the individual mandate was the key sticking point in discussions over Mr. Schwarzenegger’s plan. “It’s the biggest problem,” she said.

Mr. Obama makes a similar argument that it is unfair to mandate insurance without knowing its cost. He also contends that any individual mandate should come after affordable plans have been made available.

“First, get it out there, show people what it is,” a Harvard economics professor who is advising Mr. Obama, David Cutler, said. “Until you know you’ve got the money to pay for a generous policy or find a less generous policy people want, you’re trying to square the circle….People are afraid they’ll be shoehorned into something they don’t want.”

Advisers to Mrs. Clinton say the individual mandate must be part of reforms from the beginning, in part because such a rule will bring in healthier individuals and encourage insurers to offer reasonably priced coverage. “We believe in the individual requirement,” Mrs. Clinton’s top policy adviser, Neera Tanden, told The New York Sun. She said she’s perplexed by Mr. Obama’s opposition, especially in light of Mr. Schwarzenegger’s embrace of the idea.

“It’s surprising to us that the governor is, in many ways, to the left of Obama on this issue,” Ms. Tanden said. The Clinton campaign is already distributing fliers in Iowa attacking Mr. Obama’s lack of a mandate in his plan. TV ads going after the Illinois senator on that point are also being tested, the Associated Press reported. Ms. Tanden said she wasn’t aware of such ads.

There are major differences between Mr. Schwarzenegger’s plan and Mrs. Clinton’s proposal. “We have a premium cap that says no one will pay more than a certain percentage of their income,” Ms. Tanden said. However, Ms. Balber noted that the Clinton camp has not specified exactly where that cap would be set. “It’s almost impossible to critique her plan because there aren’t enough details,” she said. “A cap being promised without a concrete list of benefits is pretty useless.”

All the major Democratic candidates have said they would spend large sums to subsidize the purchase of insurance. Mrs. Clinton and Mr. Edwards would spend about $100 billion a year, while Mr. Obama would spend about half that.

Mr. Schwarzenegger’s proposal has divided labor groups. Some traditional unions are staunchly opposed to an individual mandate. Others, such as the Service Employees International Union, are more flexible on the point and are trying to broker a deal.

Ms. Tanden said she’s heard no criticism from labor over Mrs. Clinton’s call for a mandate. “We haven’t heard any blowback from the unions,” she said.

A conservative think tank, the Heritage Foundation, and a former Republican speaker of the house, Newt Gingrich, were early proponents of requiring individuals to purchase health insurance. However, in the current election cycle, Republican presidential candidates have shunned the idea. Even the former governor of Massachusetts, Mitt Romney, who once touted an individual mandate as part of his state’s health reforms, has soured on it.

For his part, Mr. Schwarzenegger is now dealing with a new headache. The state’s estimated budget deficit has unexpectedly ballooned to $14 billion.

“Schwarzenegger has no money,” Mr. Cutler observed. “He’s got to figure out how to put together a plan that costs nothing and gives people what they want…. You can’t do that.”