Coattails is a metaphor for one candidate being given an advantage by being associated with another, popular candidate. The image is of one candidate sliding into office by riding on the coattails of the other, more surely electable, candidate. The candidate riding on the other's coattails is generally more local, e.g., a Democratic gubernatorial candidate riding the coattails of a popular Democratic President. Coatanchor should now be obvious if you understand my hack definition and is therefore left as an exercise for the interested student.