I think the gentlemen's agreement would be to give a fair amount of time to rebuff. If someone says something that isn't true it takes time to respond because (1) the responder doesn't know what was claimed since it is a fabrication and (2) responder can't necessarily grasp the damage or interpretation of the claim until some polling data comes in.

Smears unleashed in the last week of the campaign are a bit below the belt. Doubt that is ever going to stop anyone though.