Sharpening the emerging debate over how to reshape the country's healthcare system, America's Health Insurance Plans has unveiled a proposal for covering all Americans in a more centralized insurance market. The AHIP plan would require all Americans to get coverage. In exchange, insurers would agree to longtime demands from consumer advocates that they no longer reject people with preexisting medical conditions. But consumer groups fear that a mandate requiring Americans to get insurance could force people to buy unaffordable coverage unless state or federal authorities can regulate how much insurers charge.