This paper aims at analyzing both positive and negative political campaigns which affect voters’ perception about candidate valence. Consider the situation where candidates know valences each other but the voters cannot observe one candidate’s valence. We characterize perfect Bayesian equilibria, which depend on the cost of positive campaign for the candidate and the cost of negative campaign against the opponent. The cost may be interpreted as the campaign budget constraint or the risk from backfire. We show that there always exist a pooling equilibrium and, for a wide range of parameters, a separating equilibrium. If positive campaign costs sufficiently less by the candidate with higher valence, the high valence candidate conducts positive campaign whereas the low valence candidate does not. More important, if negative campaign against the high valence opponent is sufficiently costly or risky due to backfire, the incumbent conducts negative campaign against a low valence challenger but not against a high valence one. We also show that, the smaller the valence difference is between two candidates, the larger the platform divergence becomes on the equilibrium.