Giving Americans the option of buying medical coverage through the government is a potential deal breaker for some Republicans and insurance companies whose support would ease the way for a healthcare overhaul. The proposal, which President 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% or more. 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 healthcare.