Washington ? Giving Americans the option of buying medical coverage through the government — an idea put forth by President Barack Obama — is a potential deal breaker for some Republicans and insurance companies whose support would ease the way for a health care overhaul.

The proposal, which Obama advocated in his presidential campaign, would for the first time offer government-sponsored coverage to middle-class families, as an alternative to private health plans. By some estimates, it could reduce premiums by 20 percent or more — making it much more affordable to cover the estimated 48 million people who don’t have health coverage.

But insurers fear competition from a government plan could drive them out of business, and Republicans worry it would lead to a government takeover of health care. Liberals, meanwhile, are equally adamant that Americans deserve the choice of government-sponsored health care.

Such a plan could be similar to what seniors have in Medicare, which is government run. Or it might be designed like the federal employee health plan, available to members of Congress, and delivered through private insurers.

Whatever he decides, Obama could find himself trapped between liberals in his own party and conservatives he’s trying to woo in support of a health care overhaul.

Asked about the issue at the White House health care summit this week, the president said he would address the qualms. And while saying he wanted to consider all ideas, he did not abandon the notion of a government plan.

“I’m not going to respond definitively,” Obama said, answering a question from Sen. Charles Grassley, R-Iowa. “The thinking on the public option has been that it gives consumers more choices and it helps … keep the private sector honest, because there’s some competition out there.

“I recognize, though, the fear that if a public option is run through Washington, and there are incentives to try to tamp down costs … that private insurance plans might end up feeling overwhelmed.”

Obama says he is committed to preserving a health care system in which government, employers and individuals share responsibility. Many Americans may not realize the government already picks up nearly half the nation’s $2.4 trillion health care bill, through programs including Medicare and Medicaid.

A public plan for the middle class could give a final nudge that puts the system squarely in government hands.

Obama’s campaign proposal — a foundation for Democrats in Congress — called for setting up a national insurance marketplace through which individuals and small businesses could buy coverage. People could pick private insurance or opt for a public plan that would resemble coverage for federal employees.

Sen. Mike Enzi, R-Wyo., meanwhile, said he is wary that a public plan could make insurance reforms “a sham.” His views carry weight because he is the Senate health committee’s top Republican.

“It’s important that the private market be involved, and not to set up the whole thing so it’s a sham to compete with the government, so the government eventually can be the only supplier,” Enzi said in a recent interview. “We are not going to do an expansion of Medicare. To use that as the model and try to make everybody compete with it would severely limit the market.”