WASHINGTON (AP) — Senate Democrats on Sunday sparred with each other over how to fix the nation’s troubled health care system, the moderates threatening to scuttle legislation if their demands weren’t met and more liberal members warning their party leaders not to bend.

The dispute among Democrats foretells of a rowdy floor debate next month on legislation that would extend health care coverage to roughly 31 million Americans. Republicans have already made clear they aren’t supporting the bill.

Final passage is in jeopardy, even after the chamber’s historic 60-39 vote Saturday night to begin debate.

“I don’t want a big-government, Washingtonrun operation that would undermine the ... private insurance that 200 million Americans now have,” said Sen. Ben Nelson, a conservative Nebraska Democrat.

Nelson and three other moderates agreed to open debate despite expressing reservations on the measure. Each of them has warned that they might not support the final bill.

One major sticking point is a provision that would allow Americans to buy a federal-run insurance plan if their state allows it. Moderates say they worry the so-called public option will become a huge and costly entitlement program and that other requirements in the bill could cripple businesses.

The Senate bill would require most Americans to carry insurance and provide subsidies to those who couldn’t afford it. Large companies could incur costs if they did not provide coverage to their work force. The insurance industry would come under significant new regulation under the bill, which would first ease and then ban the practice of denying coverage on the basis of pre-existing medical conditions.

Congressional budget analysts put the legislation’s cost at $979 billion over a decade.