1. A method performed by data processing apparatus, the method comprising: receiving a content item request that specifies a phrase of one or more words; determining that a first keyword and a second keyword for a same content distribution campaign are both matched by the phrase; determining that a first match between the first keyword and the phrase is a more specific match than a second match between the second keyword and the phrase; determining that the first keyword is associated with a higher bid than the second keyword; identifying, based on the determination that the first keyword is associated with the higher bid, a first tiebreaker rule specifying that the second keyword is a controlling keyword when the second keyword has a higher quality score than the first keyword; determining which of the first keyword or the second keyword has a higher quality score; selecting, based on the first tiebreaker rule and the determination of which of the first keyword or the second keyword has a higher quality score, one of the first keyword or the second keyword as a controlling keyword for the content item request; and providing, in response to the content item request, data associated with the controlling keyword.