This is only true under a better voting system than first past the post. In first past the post, you’ve got a dumbass set of broken game rules where once two parties get big enough, they become the main and only characters, and all third parties can do is debuff one of them so the other one wins.

It’s such a reliable thing that the two parties often try to fund third parties the other party’s voters will like.

Obligate games blow ass.