I've always found that using an IRC can be infinitely more effective than waiting over the course of days for candidates to post answers. With this model, you can actually get a debate in real-time, and then the moderator would just be tasked with posting a transcript here for everyone afterwards.
Also, in order to not overwhelm the candidates you could have a few debates Worth different themes.
I think Barnes is right. An IRC debate chat would be more effective and more interesting. All candidates would be able to speak at the same time. Also the moderator might open a commentary chat in order to let viewers interact.