Republican Senators responded to Senate majority leader George Mitchell's threat to work them round the clock by helping to pass the first amendment to his health-care reform bill. But the G.O.P. isn't exactly giving in. They plan to unleash a barrage of their own amendments. Meanwhile, the "mainstream coalition," a moderate group that appears to be gaining influence, met behind closed doors to fashion a sweeping package of amendments to present to both Mitchell and Senate minority leader Bob Dole.