How far can media undermine democratic institutions and how persuasive can it be in assuring public support for dictator policies? We study this question in the context of Germany between 1929 and 1939. Using quasi-random geographical variation in radio availability, we show that radio had a significant negative effect on the Nazi vote share between 1930 and 1933, when political news had an anti-Nazi slant. This negative effect was fully undone in just one month after Nazis got control over the radio in 1933 and initiated heavy radio propaganda. Radio also helped the Nazis to enroll new party members and encouraged denunciations of Jews and other open expressions of anti-Semitism after Nazis fully consolidated power. Nazi radio propaganda was most effective when combined with other propaganda tools, such as Hitler’s speeches, and when the message was more aligned with listeners’ prior as measured by historical anti-Semitism.
That is from a new and very cool paper by Adena et al (March 2013). Don't miss the graphs at the end.