The way you play such a smear is to let it build an audience, then go on TV to answer. People who do not like you, tune in to watch you squirm. But you do not squirm. You admit (in the case of the DUI) that you were not perfect at 24. That takes 20 seconds. Then you deliver a stirring 28 1/2 minute speech on your vision for America. I think you have to watch baiting and switching. I personally get tired of people who see every open mike as an opportunity to zing in the next sequential talking point, never mind how that open mike came to be there. People should already know your vision for America before election eve or you're in trouble anyway. Especially if you're an incumbent.
There is no possibility for "baiting and switching," when you are responding election eve, to an obvious smear campaign. And remember, Bush was not an incumbent in 2000--he just had the same dysfunctional political advisor.
The point is, that you let the smear build your audience. Then you can seize the moment, whether as an incumbent or candidate for new election.
William Flax Return Of The Gods Web Site