Most of the literature on grassroots campaigning focuses on mobilizing potential supporters to turn out to vote. The actual ability of partisan campaigns to boost support by changing voter preferences is unclear. We present the results of a field experiment the Australian Council of Trade Unions (ACTU) ran during the 2013 Australian Federal Election. The experiments were designed to minimize the conservative (the Coalition) vote as part of one of the largest and most extensively documented voter persuasion campaigns in Australian history. Union members who were identified as undecided voters in over 30 electorates were targeted with appeals by direct mail and phone banks. Because of the presence of compulsory voting in Australia, we are able to identify the effects of voter persuasion independently of voter turnout. We find that direct mail, the most extensively used campaign strategy in Australia, has little effect of voter persuasion. Direct human contact, on the other hand, seems to be an effective tool for voter persuasion. Among undecided voters who actually receive direct contact via phone call, we find a ten percentage point decrease in the Coalition vote. From a methodological standpoint, we use various methods to account for multiple treatment arms, measured treatment noncompliance in one of the treatments, and missing outcome and covariate data. The field experiment also provides a good lesson in conducting and saving broken experiments in the presence of planning uncertainty and implementation failures.