This paper sheds light on the links between media and political polarization by looking at the introduction of broadcast TV in the US. We provide causal evidence that broadcast TV decreased the ideological extremism of US representatives. We then show that exposure to radio was associated with decreased polarization. We interpret this result using a simple framework that identifies two channels linking media environment to politicians’ incentives to polarize. First, the ideology effect: changes in the media environment may affect the distribution of citizens’ ideological views, with politicians moving their positions accordingly. Second, the motivation effect: the media may affect citizens’ political motivation, changing the ideological composition of the electorate and thereby impacting elite polarization while mass polarization is unchanged. The evidence on polarization and turnout is consistent with a prevalence of the ideology effect in the case of TV, as both of them decreased. Increased turnout associated with radio exposure is in turn consistent with a role for the motivation effect.