[RebelBanker]

Let us assume a very simple case: 2 candidates ("A" and "B") and 100 voters (evenly divided 50-50). If another candidate ("C") comes along and takes 5 votes from B, the vote is now 50 for A, 45 for B and 5 for C. Thus, A wins. This is a pretty straight forward divide and conquer strategy.

Of course, the real world is a lot more complicated than that, but I believe that concept holds up under our system. It is quite different under a parliamentary system (like in Italy or Israel) where major parties need minor parties to form coalition governments so the "small fry" actually get a disproportionately large voice.

[MrLeRoy]

If another candidate ("C") comes along and takes 5 votes from B

This assumes those five voters would otherwise have held their noses and voted for B rather than simply staying home. And while it may be an argument for third-party candidates to not enter the race, it does not address how voters in a three-candidate race should vote.