
McCain
McCain needs to project a sense of competance. The biggest issue facing him is this idea that Republicans are no longer competant. Given the terrible state of the economy, plus the dismal failures of the Bush administration in dealing with foreign affairs, Iraq, Katrina, etc., McCain must reassure voters that he is more competant than any other Republican out there. If he can come off strong, self-assured, and competant, he'll do fine tonight.
Obama
Obama needs to demonstrate his commander-in-chief"iness." The biggest issue facing Obama is convincing voters that he can stand up and be trusted to face the daunting issues that face this country - see above. If he can show that he can stand toe-to-toe with McCain, and people feel that he passes the commander-in-chief threshold, Obama can't lose this election.
Bottom-line: Both this debate and the election are Obama's to lose.