We investigate the novel problem of voting-based opinion maximization in a social network: Find a given number of seed nodes for a target campaigner, in the presence of other competing campaigns, so as to maximize a voting-based score for the target campaigner at a given time horizon. The bulk of the influence maximization literature assumes that social network users can switch between only two discrete states, inactive and active, and the choice to switch is frozen upon one-time activation. In reality, even when having a preferred opinion, a user may not completely despise the other opinions, and the preference level may vary over time due to social influence. To this end, we employ models rooted in opinion formation and diffusion, and use several voting-based scores to determine a user's vote for each of the multiple campaigners at a given time horizon. Our problem is NP-hard and non-submodular for various scores. We design greedy seed selection algorithms with quality guarantees for our scoring functions via sandwich approximation. To improve the efficiency, we develop random walk and sketch-based opinion computation, with quality guarantees. Empirical results validate our effectiveness, efficiency, and scalability.