Democrats have decided that unless there is an unexpected development in the weeks ahead, they will not launch a major fight to block the Supreme Court nomination of John G. Roberts Jr., according to legislators, Senate aides and party strategists. In a series of interviews in recent days, more than a dozen Democratic senators and aides who are intimately involved in deliberations about strategy said that they see no evidence that most Democratic senators are prepared to expend political capital in what is widely seen as a futile effort to derail the nomination.