Half of Americans do not identify with either major party. They are not nearly as "split" as the partisans that try to control the political discourse.
Yet most unaffiliated voters “lean”, often very strongly, towards one party or the other. Most estimates put true independent/swing voters at only 3-4% of the electorate.
There's a big equivocation between your post and GP's: "American" => "voter". But people who are not well-represented by the two major parties are probably less likely to vote.
If they're not Americans then how are they voting?