The candidates' plans differ in some important provisions, and certain key details are not yet spelled out. But Clinton, Obama and Edwards follow the same basic framework.
Private insurers who wished to participate in the pools would not be allowed to turn people down because of previous health problems, and the insurers would be limited in how much they could raise premiums to account for a person's age and other factors that can affect health. All of the plans in the pool would have to offer a minimum standard benefit package, which would probably be keyed to the government plan.
Individuals and families eligible for government subsidies through the candidates' health plans would be able to use that assistance to buy either a private plan or the government's.
Businesses would be allowed to participate in the purchasing pools and enroll employees in the government plan. However, in Obama's plan, participation would be limited to small firms and individuals who don't have access to employer coverage.
The question of how a government plan would fare in competition with such well-known brands as Blue Cross Blue Shield, Kaiser Permanente and Humana is a matter of intense speculation among policy experts and people in the healthcare industry. It's generally assumed that the government plan would cost less than many of the private options. But it's not clear whether that would necessarily give it an advantage.
Government coverage "could become the fallback plan for sick people," said health economist Jack Rodgers of the consulting firm PricewaterhouseCoopers. "If you end up with a federal plan that has a lot of sick people in it, is that something that you would want the federal government to subsidize, or would it be doomed?"
Most healthcare costs are incurred by the sickest patients. In any given year, about 10% of the population accounts for more than 60% of healthcare costs -- a phenomenon that partly explains why insurers go to great lengths to avoid issuing policies to people with medical problems.
Edwards' policy director, James Kvaal, said Edwards would insist on regulations to prevent private carriers from dumping their sickest patients into the government's care. "We will stop private insurance companies from competing by weeding out the sickest people," Kvaal said.
But what if those patients -- weary of dealing with private insurers -- sought out the government plan as a haven? Rules for insurers might not protect the government plan in such a situation. Instead, lawmakers might have to wade into the complex business of trying to adjust premiums between the government plan and the private policies.
The candidates acknowledge that their plans are broad brush strokes for now, and important details would have to be decided with Congress. Nonetheless, the consensus among the three means that if a Democrat is elected, a new government insurance plan for the middle class could well be part of the strategy for tackling one of the nation's most worrisome problems.
"Sen. Edwards has heard from a lot of people on both sides," Kvaal said. "Some people feel very strongly that without the profit motive, healthcare could be offered at higher quality and lower cost. Some people feel that private plans are more nimble and innovative. Offering both in competition gives families a choice."