Actually, I thought Kerry did pretty good tonight. Chris Suellentrop in Slate said that in Tuesday's speech Gore hit the third gear he usually skips between robot and mental-patient. The appropriate driving metaphor for Kerry's speech tonight is: he stayed on the road.
It's often observed that a presidential re-election is a referendum on the incumbent. So, Kerry has to not seem risky or erratic and keep his negatives down. This is what kausfiles calls the Eddie Yost strategy (after the Red Sox player who became an All Star by leading the league in walks and hence time on-base).
If Kerry manages it, though, with any luck G.W. will defeat himself.