We report the results of a field experiment conducted in New York City during the 2013 election cycle, examining the impact of nonpartisan messages on donations from small contributors. Using information from voter registration and campaign finance records, we built a forecasting model to identify voters with an above-average probability of donating. A random sample of these voters received one of four messages asking them to donate to a candidate of their choice. Half of these treatments reminded voters that New York City's campaign finance program matches small donations with public funds. Candidates’ financial disclosures to the city's Campaign Finance Board reveal that only the message mentioning policy (in generic terms) increased donations. Surprisingly, reminding voters that matching funds multiplied the value of their contribution had no effect. Our experiment sheds light on the motivations of donors and represents the first attempt to assess nonpartisan appeals to contribute.