Let's say your preference is Green over Democrat and Democrat over Republican. Under approval voting, I should vote for Nader and Gore. But what if the Green Party slowly turns into a major party while the Republicans decline? Voting for both Democrat and Green makes sense only if the Republican has a chance of winning. Once the Republican slips in popularity, then you are faced with dilemma of, "When should I stop voting for the Democrat?" Wait too long, you might help Gore beat Nader. If you commit too soon to Nader, you might help Bush beat Gore.