Political parties in US politics are becoming increasingly polarized, with a growing number of extreme candidates entering electoral races. Why would extremists challenge more moderate opponents, since their chances of winning are supposedly very slim? I develop a model of electoral competition and endogenous entry to show that extremists rely on the possibility that the campaign might reveal information about the opponents’ quality that can induce some voters to change their electoral decision. The weight voters place on candidates’ valence determines the incidence of uncontested elections and the degree of asymmetry in polarization of the candidates’ policy positions in contested elections. Finally, I extend the model to consider asymmetric information about individual valence levels. I show that uncontested races are still possible in equilibrium, that in contested races at least one candidate has high valence, and the valence-advantaged candidate can be the one with the more extreme policy stance.