Senate Republicans, in a move that could seize the deficit-cutting initiative from more radical House members, unveiled proposals for nearly half a trillion dollars in painful spending cuts by the year 2000. A Senate G.O.P. task force, led by Sen. Judd Gregg of New Hampshire, seeks to force $475 billion in cuts from massive entitlement programs: $89 billion from projected welfare costs of $600 billion, and as much as $275 billion from Medicare and Medicaid, which are now projected to cost $1.82 trillion by the decade's end. Also included: a scheme to shrink cost-of-living increases to Social Security recipients. The report, in a barb at opponents who demanded to know where the GOP cuts would fall, challenged Democrats "to detail their alternatives, or to explain what taxes they would raise, where they would cut Social Security or why their believe the federal budget should not be balanced."