From what I have heard, the undecideds are not "a large part of the American public" either but represent a very small percentage of the voting public.

But from what I've heard, the voting American public is a pretty small percentage of the actual American public of voting age - I'd have said if you don't vote you're undecided. (There are in fact quite a few countries where, if less than half the electorate voted, they'd have to rerun the election.)