Transcript for Looking ahead to crucial 1st presidential debate
And for more on tonight's debate let's bring an ABC news political analyst Matthew Dowd Matt certainly a big night tonight so let's break this down on both sides. What do you see as the most important thing that Joseph Biden needs to accomplish tonight in what is president trump need to do to have a strong showing. Well Lindsey it is a big night probably going to be that most watched event of this whole cycle. Upwards of seven year eighty million or more. In this I think Joseph Biden has to be come across as calm cool collected and thoughtful present his case about why wants to be present a United States prosecute the case against. President trump about what he's done over the last four years. And why he thinks it's been not good for the American public so I think he's been spending be clear concise and there may be a moment that he has to come back and pushed back strong against. An attack from Donald company should be that limited Donald Trump money. On the other hand I think should stay away from a correct constant attack. I'm Joseph Biden he probably won't help you can't help himself he probably will but the best thing for down trop. Is to present the reason why his presidency has been good for the American public. Presented in a way. That rehabilitate his image because that's the biggest thing it's holding him back right now stop the perception of Joseph Biden is a perception of the. Donald Trump. About 14% of Americans still identify themselves as undecided voters with so many voters have already made up their minds about these candidates are real target audience tonight. And can this debate really change much. So to me you and I look at this debate there's a small segment of that the country that's undecided. It's really tiny it's true undecided voters there's a lot that leaned. And then there is the voters that are with Joseph Biden did Donald Trump passed to move to him because right now Donald Trump comes into this. By all accounts seven or eight points behind national and hide in every key. Target state in the Electoral College so Donald Trump has to move those voters Joseph Biden has to reassure his voters that are with them already. And try to speak to some of those small segment of undecided voters. I think debates are the last thing that can really move in this race this debate in the ones that follow are they move they move the race in 2016 Hillary. Clinton came into that first debate two points up she left that first debate five points up the race ended up to be a two point race. So that's the group of voters in both should be targeting the same group of voters undecideds. And sop Biden voters because Donald Trump has to move voters away from Joseph Biden in order to win. Really want to piggyback off that point that you just made because as you said and most analysts saw the Hillary Clinton won the debates in 2016 she still ended up losing raised. So is there a difference this year. Compared to 2016. As far as the the potential impact of this debate could have. If there's a huge difference there's a big difference between race said it's an open race it becomes a choice election and that's what Tony sixteen was it was an open race. That you had decide between the two of them. This race is more referendum election or referendum on the president the other big difference. Is that there is really no significant third party players. In this race as there was last time five or 6% above vote on Election Day went to somebody else other than Donald Trump. Our Hillary Clinton and so that's a huge difference in this race I think the debate really matters in that context because. It becomes a referendum on Donald Trump and met and voters wanted to decide OK and we decided that the country is not headed in the right direction. Do we want to replace Donald Trump. With Joseph Biden and that's a big difference from from point a sixteen a major difference in a referendum election and a choice selection which is what. This year is a dream this year is a referendum on the last four years. Matt down thanks so much will he talk until later on tonight. Sounds great Lindsay thank you. Again that crucial first debate just ahead stay tuned right here on ABC news live for a special edition of 20/20 starting at 8 PM eastern followed by the debate. At 9 PM.
This transcript has been automatically generated and may not be 100% accurate.