Say you lived in a society with a First Past the Post voting system. Say at the last election the Evil Party won with 11 million votes. Second came the Slightly Less Evil Party with 10 million votes. After them came a variety of Not At All Evil parties with 9 million votes.
How should you vote this time?
If you vote tactically, as millions of your fellow citizens did last time and do in every election, you should vote Slightly Less Evil. After all, if the only possible results are Evil or Slightly Less Evil choose the lesser.
But it is this tactical calculation that both Evil parties rely upon. If nobody voted tactically at all, and instead voted for the best of the Not At All Evil parties on offer, one of them would win. Maybe not this time, but at some point in the future.
The problem with tactical voting in situations like this is that we can all get locked into a choice between Evil and Slightly Less Evil. Even if we think that we should determine our vote by some kind of utilitarian calculus, supporting one form of Evil where it is less bad than the alternative (I do not), longterm tactical voting is counterproductive. Longterm the best tactic is not to vote tactically this time. Elections are not one offs, and tactical voting may leave us trapped between two awful options forever.
Vote Not At All Evil.
[Any relation of this post to any real world choice in the 2019 UK General Election is purely coincidental.]