And with the race this close, so many observers noting just how much this third debate in this hall will matter this time. And with the race a dead heat, just a half percentage in the polls one way or... See More
And with the race this close, so many observers noting just how much this third debate in this hall will matter this time. And with the race a dead heat, just a half percentage in the polls one way or another could really swing this race. And among those also watching tomorrow night, the undecided voters in this country. We've been hearing them as we travel through the battlegrounds with 16 days to go. Tonight here, undecided voters here in florida, and we ask just how few of them could make a real difference? The big debate tomorrow night. How much can they shift opinion in these polls with the last debate? Well, the conventional wisdom that debates don't matter was shattered pretty well by the denver debate. Reporter: Nate silver tracks it all, every poll number, every shift in his 538 blog for "the new york times." Constantly running the numbers, averaging the polls in every key battleground state. And with the national race now tied tonight, silver says right now, the president's numbers in the midwestern battleground states give him a very slim edge, which is why both candidates this evening are concentrating on that last chance on the stage to reach the undecideds. How many undecideds in this final debate? So, the average poll shows only about 5% of voters who are still undecided. Reporter: Who are they? Who is undecided at this point? They are -- not a lot of people in the country who think they have two terrific choices. The undecideds are people that are not persuaded that romney is not a good alternative but disapprove with obama's performance in the white house. Reporter: Here in florida, we went looking for the undecided voters. And they're not easy to find. Watching the debate tomorrow night? Absolutely. Yes. Reporter: Two of the three have decided, but michelle, in the middle, has not. Still not convinced whose economic plan is most fair. They need to help people more. Focus on the people falling through the cracks. Reporter: You are voting president obama, you're voting mitt romney. You're going to be the decided vote. Yes. Reporter: Let's check back in with you. Frank, leaning obama, but says romney in that firstebate easterned a second look. I would be for obama, but mr. Romney brought up a lot of good issues. Reporter: So, you really are going to be watching closely? Absolutely. I think a lot of people are. Reporter: In fact, convincing the few undecided voters that remain could change the race. Just ask nate. Says tomorrow night the candidate that moves the needle even by a fraction could win. If one candidate gets a half a point gain, even a one-point gain, because it's so close, that could swing the election. A half a point. Incredible. Nate has a new book out, "the signal and the noise." His numbers are updated constantly. I'm checking it right now, 68% chance of winning for president obama, if the election were held right now. That's because of the slight advantage for him in the battleground states. Everyone will be watching this tomorrow night. Hope you watch it right here on abc news. We begin tomorrow night with george and diane in new york, along with the rest of the power house political team. I'll be hereith jake tapper. 9:00 p.M. Eastern, right here in the hall.
This transcript has been automatically generated and may not be 100% accurate.