the idea that there are 'undecided voters' is a bit of a myth.athe vast majority of people just vote based on their identity and what 'team' they think they are on. They don't really care about the quality of the candidates and they don't even care if their candidates keep their promises.
the only thing that the normal people really react to is signaling from whatever media they consume (and the media they consume is determined by their identity). So if CNN starts saying biden is unfit, then people will believe it. They will trust CNN messaging more than their own eyes.