Craig Burnett and Vlad Kogan have posted this draft on SSRN.  Here is the abstract:

At last count, U.S. voters were responsible for choosing more than 510,000 distinct elected officials. Few of these contests feature lively campaigns or attract substantial media attention, forcing voters to make decisions with limited direct knowledge of the candidates. What strategies do voters use to do so? Using a “Who Said What?” experiment fielded during an election in a major American city, we show that voters engage in social categorization — and do so on the basis of race and ethnicity when candidates differ in their demographic background. We also find, however, that the degree to which they categorize candidates on this dimension depends on the presence or absence of party labels. Our results suggest that efforts to increase minority representation should look beyond electoral institutions such as district vs. at-large elections to the structure of the ballot itself.