I'm not saying that assumption is wrong. But I'm not comfortable with it, either, because of the other assumptions that go with it. It implies that the qualitative content of political messages doesn't matter -- that voters' choices are driven by how many messages from Candidate X they see and hear,, not by fewer but more cogent, persuasive messages from Candidate Y. It implies that campaign advertising outweighs following and reflecting on the the news... talking politics with friends... comparing what the candidates say in this ad with what they say in other ads or to other audiences, and what they've said and done in the past.
lt implies passive, stupid voters. And to the extent that your crtitique of money in politics takes that for granted, your democracy has worse problems than Charles and David Koch.