L Eveling the Playing Field : H Ow Campaign Advertising Can Help Non - Dominant Parties ∗

We examine how campaign advertising affects electoral support. We propose a simple model where advertising disproportionately benefits non-dominant political parties, because voters are uncertain about and biased against such parties. We test this argument in Mexico, where one of the three main parties dominates in many localities. To identify the effects of exposure to campaign advertising, we exploit differences across neighboring precincts in campaign ad distribution. These differences originate from cross-state media coverage spillovers induced by a 2007 reform that equalized access to ad slots across all broadcast media. We find that, on average, ads on AM radio increase the vote shares of the PAN and the PRD, but not the previously-hegemonic PRI. Consistent with our model, campaign advertising is most effective in poorly informed and politically uncompetitive electoral precincts, and against locally dominant parties of intermediate strength.