Where are these 20% undecided voters? If we assume they are spread evenly throughout the country, then none of them will have any meaningful impact in the state they are voting in.
The electoral college is an all or nothing system. Even if all undecided voters in a red state decide to vote for Hilary, the state would still go to Trump, and vice versa in a blue state.
I've been hearing about this 20% for many many years. I wonder if it ever really existed as such a large block of voters, or are the Networks trying to keep us on the edge of our seats.. Even if that 20% was true at some point in time, in this ever increasingly partisan country, I don't think that 20% holds true anymore - or has a study been done recently that confirms that the often quoted 20% is still valid?
Unless I can see something that validates that number, I'm going to take the position that it's one of those things that someone said once, and was repeated so many times, it became the "truth".