Deep fissures among Senate Democrats became evident on two issues as lawmakers moved closer to a floor debate on legislation to remake the healthcare system: whether the government should sell health insurance in competition with private insurers, and whether Congress should offset any of the cost of legislation to increase Medicare payments to doctors. The Senate majority leader, Harry Reid, Democrat of Nevada, said that he would try next week to pass a free-standing bill that would pour $240 billion into Medicare to shield doctors from cuts in their Medicare payments that would otherwise occur, under existing law, in the next 10 years.