Robert, I definitely understand what you are saying and know that there can be no leaks. But in the mean time your team could work with the candidate on his online reputation management. By creating non campaign media and content and optimizing it for the targeted keywords which will be the candidates full name.
A great example of a candidate who failed at this preparation was GOP presidential candidate Rick Santorum. Danny Sullivan wrote about here. For four years leading up to his announcement to run, his campaign should have had a team working on his online reputation management problem. So that by the time he announced it, his name would have owned the first three pages of the SERP's. Instead, for most of his campaign the number 2 result on Google was a website defining Santorum as something disgusting that you wouldn't want your grandmother to find and negatively affecting his online presence. For most of the campaign there were two other negative sites that also showed up on the first page for his names SERP. If you ask me, I believe it could have been avoided.
I understand that your possible candidate probably isn't as high profile but that doesn't mean it won't get ugly during the race. I believe it would be better to own the SERP's now than have to try to play catch up. I hope that is helpful.