Democrats say the proposal would undermine traditional Medicare.

Congress should make changes in the drug benefit, instead of letting it serve as an opening wedge to privatizing and thus undermining original Medicare.

Democrats hope to use the contract as a stick to beat Republicans, saying they have undermined Medicare, welfare and environmental programs.

House Democratic leaders pressured members of their party to vote against the bill, which they said would undermine and privatize Medicare.

They say it may undermine Medicare by removing relatively affluent retirees from the program and leaving the poor.

They say it would undermine traditional Medicare, which serves 88 percent of the 40 million beneficiaries.

"This so-called premium-support proposal, which is really a voucher plan, would undermine Medicare as a social insurance program," Professor Marmor said.

He said the legislation would undermine Social Security and Medicare.

Conservatives see it as a way to slow spending, but many Democrats say it would undermine Medicare.

"It's the first phase of undermining Medicare as we know it."