Democratic senators would oppose a potential House bipartisan deal preventing cuts in physicians' Medicare payments if it doesn't finance a children's health program for four years, senior Senate Democratic aides said Sunday.

The aides issued the threat as House bargainers try preventing a 21 percent reduction in doctors' Medicare reimbursements scheduled for April 1. Negotiators want to craft an agreement annulling a 1997 formula that annually threatens deep cuts in those payments.

According to lobbyists and congressional aides, bargainers have discussed including money for the Children's Health Insurance Program for two more years, costing around $5 billion. Its money is due to expire Oct. 1.

It was unclear if the Democratic aides' comments were a firm line in the sand or negotiating pressure. Either way, they underscored how contentious it will be for the Medicare package to quickly clear Congress.

The children's program is supported by both parties. Congressional Democrats want its renewal included in the Medicare deal, which is viewed as one of the few health measures that could be enacted this year.

The overall agreement -- eliminating the Medicare reimbursement formula, slightly boosting those payments and adding money for children's health -- would cost roughly $200 billion over 10 years, aides and lobbyists say.

About $140 billion of the measure's cost would be added to federal deficits and has already drawn opposition from many conservatives. The rest would be split between increased costs for Medicare providers and beneficiaries.

Many congressional Democrats could oppose the deal because of its price increases for recipients. One proposal would make buyers of Medigap policies -- for costs Medicare doesn't insure -- pay the first $250 before coverage begins.

The aides and lobbyists spoke on condition of anonymity to discuss private negotiations.

Republican aides declined comment.