To measure this maximum potential, we answer the question, “assuming that the only goal of a media owner were to favour one political party, and that media users took the information they receive at face value, how much could the media owner influence voting behaviour?” The answer to this question identifies an upper bound to the power of a media company. It provides a basis for prudential regulation by directing attention to the highest damage potential of any media ownership pattern.3 Define the attention share that a voter devotes to a particular news source as one divided by the number of sources that voter uses, and zero if he does not follow that source. Voters that focus on a narrower selection of sources will thus have large attention shares attached to each source. The overall attention share of each source is the average attention share by source over all voters. Within this framework the power of a media company is a simple function of its overall attention share.