tactical voting

Strategic voting, also called tactical voting, sophisticated voting or insincere voting, occurs in voting systems when a voter votes for another candidate or party than their sincere preference to prevent an undesirable outcome. For example, in a simple plurality election, a voter might gain a better outcome by voting for a less preferred but more generally popular candidate. Gibbard's theorem shows that all single-winner voting methods encourage strategic voting, unless there are only two options or dictatorial (i.e., a distinguished agent exists who can impose the outcome). For multi-winner elections no general theorem for strategic voting exists. Strategic voting, even under proportional representation systems, is observed due to non-proportionality, electoral thresholds and quotas. But in systems that use ranked voting, there is no need for strategic voting, and under STV it is impractical to engage in strategic voting. From Wikipedia