Kerry answered first and unhesitantly replied "Nuclear proliferation." and then went into a detailed explanation about, and plan for dealing with, the loose nukes problem. Bush agreed, but then insisted on tacking on "in the hands of a terrorist" on. Then, after they'd each had their say...
LEHRER: Just for this one-minute discussion here, just for whatever seconds it takes: So it's correct to say, that if somebody is listening to this, that both of you agree, if you're reelected, Mr. President, and if you are elected [Sen. Kerry], the single most serious threat you believe, both of you believe, is nuclear proliferation?
BUSH: In the hands of a terrorist enemy.
KERRY: Weapons of mass destruction, nuclear proliferation.
Bush's needless (in this case) fixation with the terrorist label is laughable. It's obviously implied in this instance, so harping on it is unnecessary, but it also betrays the folly of his Missle Defense fiasco.
If "weapons of mass destruction in the hands of terrorists" is the threat, what the hell is the missle shield for? Is Bush actually worried about Osama building his own ICBM with his purloined nuclear material? Or is a dirty bomb / shipping container a more realistic threat. Because he's doing jack shit about that. (Though I should confess the Missle Defense System will be every bit as effective intercepting suitcase bombs as incoming missles...)
This man is an idiot, and his plans for Homeland Security are a joke. Bring it home, Big John.