Leading Congressional Democrats immediately recoiled Tuesday from a new proposal to cut $600 billion in Medicare spending over the next decade — in part by raising the eligibility age. Sens. Joseph I. Lieberman (I-Conn.) and Tom Coburn (R-Olka.) unveiled the proposal as part of a bipartisan effort to produce the kind of savings necessary to achieve the $2 trillion in debt reduction both parties say is needed to convince reticent lawmakers to vote to raise the debt ceiling.