The White House and Senate Democratic leaders seem willing to give Senator Ben Nelson, Democrat of Nebraska, just about anything he wants to win his support of major healthcare legislation. Anything, that is, but the item at the top of Nelson's wish-list: air-tight restrictions on insurance coverage for abortions. The bid to win Nelson's support has become a race against the clock. The Senate majority leader, Harry Reid of Nevada, has developed plans for a series of votes beginning at 1 a.m. Monday and round-the-clock Senate sessions intended to meet his deadline of completing the healthcare bill before Christmas. But Reid is still at least one vote short of the 60 he needs to move the bill ahead, and as much as anyone, Nelson appears to hold the legislation's fate in his hands.