Is there any evidence to indicate that mail-based campaigns make a real difference at the ballot box?

Yes! Political scientists have conducted dozens of experiments on the effect of direct mail and find that, on average, mailers increase turnout when they use “messages that emphasize civic duty or the importance of making one’s voice heard” (Gerber and Green 2015, pg. 58). Additional evidence also shows that making the interaction feel personalized, rather than just a blanket message from an organization, is key to increasing the effectiveness of voter outreach.