The Sierra Club endorsed Hillary Rodham Clinton (D) yesterday in the hotly contested race to represent New York in the Senate. The move is viewed as a setback for Clinton’s opponent, Rep. Rick Lazio (R), who won the group’s endorsement in his 1996 House race and fought hard to win it this time. Enviros have praised Lazio for his efforts to clean up Long Island Sound and protect natural resources, but the Sierra Club said yesterday that he has “dropped the ball” on a number of important green issues and supported parts of the “Contract with America” that would have stripped away environmental protections. Sierra Club officials conducted interviews with both candidates and concluded that Clinton is better informed on green issues. Lazio’s campaign contended that he has a strong environmental record while Clinton has no record at all.