15. The system of claim 14 , wherein the instructions cause the one or more data processing apparatus to perform operations comprising: receiving a second content item request that specifies a second phrase of one or more words; determining that two keywords in a same content distribution campaign are both eligible to be selected as a controlling keyword for the second content item request; identifying a tiebreaker rule based on data associated with the two keywords; identifying, from the two keywords, a second controlling keyword based on the tiebreaker rule; and providing, in response to the second request, data associated with the second controlling keyword.
15. The system of claim 14 , wherein the instructions cause the one or more data processing apparatus to perform operations comprising: receiving a second content item request that specifies a second phrase of one or more words; determining that two keywords in a same content distribution campaign are both eligible to be selected as a controlling keyword for the second content item request; identifying a tiebreaker rule based on data associated with the two keywords; identifying, from the two keywords, a second controlling keyword based on the tiebreaker rule; and providing, in response to the second request, data associated with the second controlling keyword. 20. The computer readable medium of claim 15 , wherein identifying a second controlling keyword based on the tiebreaker rule comprises: determining that a first keyword from the two keywords has a less specific match with the phrase than a second keyword from the two keywords; determining that a first quality score for the first keyword exceeds a second quality score for the second keyword; associating, with the first keyword, an adjusted bid that does not exceed a maximum bid associated with the second keyword; ranking the first keyword and second keyword based on selection scores, a selection score for the first keyword being based on the adjusted bid and the first quality score, a selection score for the second keyword being based on the maximum bid and the second quality score; and selecting, as the second controlling keyword, a highest ranking keyword.