Leveling the Playing Field: How Campaign Advertising Can Help Non-Dominant Parties

Voters are often uncertain about and biased against non-dominant political parties. By reducing the information gap with dominant parties, political advertising may thus disproportionately benefit non-dominant parties electorally. We test this argument in Mexico, where three main parties dominate many localities. To identify the effects of exposure to partisan advertising, we exploit differences across neighboring precincts in campaign ad distributions arising from cross-state media coverage spillovers induced by a 2007 reform that equalized access to ad slots across all broadcast media. Our results show that ads on AM radio increase the vote shares of the PAN and 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.