Ballot initiatives are consequential and common, with total spending on initiative campaigns in the US rivaling that of Presidential campaigns. Observational studies using regression approaches on observational data have alternately found that initiative campaign spending cannot affect initiative outcomes, can increase the number of votes rejecting (but not approving) initiatives, or can affect outcomes in either direction. We report the first well-powered precinct randomized field experiment to evaluate an initiative advocacy campaign. We find that campaigns can influence both rejection and approval of initiatives by changing how citizens vote, as opposed to by influencing turnout or ballot completion. Our experiment (involving around 18% of Oregon households in 2008) studied a statewide mail program conducted by a Political Action Committee. Results further suggest that two initiatives would have passed if not for the advocacy campaign to reject them. We discuss implications for theories about direct democracy, campaign finance, and campaign effects.