Hillary Rodham Clinton, Barack Obama and John Edwards have been sniping at each other for months over healthcare, but there's one thing the top Democratic presidential candidates agree on: Americans of all ages should have the choice of buying a government-run plan modeled on Medicare.

The idea, which would set up a competition between a new government plan and private insurance programs, has been overshadowed by the political horse race. But it's one of the most far-reaching and controversial proposals for making health insurance more affordable and more widely available.

The government now guarantees access to healthcare only for seniors and the disabled through Medicare and for the poor mainly through Medicaid. Under the proposals being advanced by Clinton, Obama and Edwards, the government would offer coverage for middle-class workers and their families, with benefits comparable to those now provided for federal employees and members of Congress.

Participation in the government plan would be voluntary, but the approach sparks widely differing reactions.

Mark McClellan, the former Medicare administrator in the Bush administration, called it a risky departure from the state-based changes that the Democratic candidates have cited as their models.

"Of all the states that are considering [reforms], none have set up a public plan to compete with private plans," McClellan, now with the Brookings Institution public-policy center, noted. "It could end up being a back door to single-payer," he added, referring to a government-run system such as Canada's.

What McClellan and other critics say they fear is that the government plan could underbid private insurers and, if large numbers of people sign up, it could eventually replace private coverage, including the employer-sponsored plans that now serve most middle-class Americans.

Some also fear that the government program would become a dumping ground for the most seriously ill -- and expensive -- patients as private companies cherry-picked those least likely to file large claims. If that happened, the federal program's cost would soar.

Despite these concerns, other experts say that giving people the option of joining a government plan might make for a sensible experiment.

"It could be a really fair way to test the market, both in terms of people's preferences and to see what kind of plan can be more successful over time," said health economist Marilyn Moon, a Medicare and Social Security trustee during the Clinton administration.

For example, Moon suggested that a government plan could be used to test ideas for reducing waste and improving quality, such as cutting payments for treatments of dubious value and rewarding hospitals and doctors that document improved results in patient care.

"You could allow a government option as a way of setting a standard, as opposed to issuing lots of regulations that would apply to every insurer," she said.

Republican presidential candidates also have healthcare plans, but -- in keeping with conservative principles -- they rely on market forces, economic incentives and the private sector to increase access to medical insurance and to try to rein in costs. None favors creation of a government alternative to private programs.

GOP opposition is not the only political obstacle to the Democratic idea.

The insurance industry, one of the nation's most powerful lobbies, is against it. "It is a move to significantly expand the role of government and move people off private coverage and to public coverage," said Karen Ignagni, president of America's Health Insurance Plans and the industry's top lobbyist in Washington.

Another critic is GOP presidential candidate Mitt Romney, who as governor of Massachusetts signed bipartisan legislation to provide coverage for all state residents -- a model for the Democratic candidates.

"You can call [the government plan] an option, but if you don't put any restrictions on who can enroll, and you give the government leverage in terms of ability to negotiate prices, you create a situation that leads to a government-run system," said Lanhee J. Chen, Romney's domestic-policy advisor.

Clinton, Obama and Edwards dismiss the charge that they are trying to introduce the camel's nose of government-run healthcare under the tent of health reform.

"Hillary never supported a single-payer system, not even in the early 1990s," said Neera Tanden, Clinton's health policy advisor. "She has always supported a plan to build on what we have in our healthcare system." (Only one Democratic candidate, Rep. Dennis J. Kucinich of Ohio, favors switching to a full-blown government-run system.)