The reason why I would expect it to apply to political parties, is because it does seem like both parties are competing for the same votes. I would expect any individual to vote for the candidate that best represents them.

So your suggesting that individuals aren't just voting for the party/candidate that most closely matches their viewpoints. Rather that there are two (or more) separate groups that will only vote for a candidate in their group. Or phrased another way, if there were two candidates, both in the very left, an individual wouldn't vote for the candidate that was more to the right, because both are too left for them.