These were the make or break moments of past presidential debates. Can Harris and Trump avoid them?

I Social Security payroll contribution will go up, as will Donald's, assuming he can't figure out how to get out of it. but what we want to do is to replenish the such a nasty trust fund. Well, that was one of the many unforgettable moments from the 2016 debate between Donald Trump and Hillary Clinton. For decades, presidential debates have been a cornerstone of American politics. They can make or break a candidate, letting the public judge appearance, policy and personality. Well, that's certainly how it's been to date. CNN's Sunlen Serfaty takes us through some of the most pivotal debate moments in American history. We're going to call you Donald Duck. It is a make or break moment, but you can't name the third one. The third agency of government, I would I would do away with the education. the, commerce of commerce. And let's see, I can't. The third one I can't. Sorry. Oops. The debate stage producing some of the most memorable, insignificant turns of the campaigns. Senator, I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy with break out moments that have upended the race. I am not going to exploit for political purposes my opponent's youth and inexperience. Given a boost of attention to lesser known candidates. I think women all over this country heard very clearly what Mr. Trump said, helping define them and their campaign. They say the difference between a hockey mom and a pit bull lipstick. But for all the highlights, it is the last opportunities and blunders that often are the most enduring. If Kitty Dukakis were raped and murdered, would you favor an irrevocable death penalty for the killer? No, I don't, Bernard. And I think you know that I have opposed the death penalty during all of my life. Snippets that are later played and replayed. He's very likable. I agree with that. I don't think I'm that bad. you're likable enough. Thank you. Sticking to the candidate. Well, after the debate is over, I went to a number of women's groups and said, can you help us find folks? And they brought us whole binders full of women. The optics of the big stage has also tripped up many candidates with a casual glance at a watch. An audible sigh. Man's practicing fuzzy math again. There's differences or even body language. But can you get things done? And I believe I can. All attempting to distract voters from the message. Sunlen Safadi, CNN, Washington. Well, joining me now to discuss what you may expect from tonight's debate is CNN political analyst, historian and Princeton professor, a good friend of this show, Julian Zelizer, televised debates. And it's good to have you. Televised debates in the United States do hold a unique position. In the past, they have had the ability to make or break a candidate based upon appearance and performance. As we most recently saw with President Biden and the abject performance in the CNN way back at the end of June. As a historian and political analyst, how much of an impact does a policy aspect of the debate make versus the visual representation, the optics or performance that much? I mean, the optics are what matter. I think the way to think about the debates is you're watching the demeanor of candidates. You're trying to glean a little bit about the character through how they act, through their demeanor, through their interaction, and simply through the speed in which they can think of a response to an attack or launch their own counterattack. These are not events where ultimately voters are going through the fine, detail of policy, which isn't even discussed. So I think that is the reality frustrates some people, but that's ultimately the metric through which we evaluate how they do. It's been a number of studies over the years, that have looked at the impact of presidential debates on voting behavior and on polling a University of Chicago and University of Austin study looking at debates and opinion polls from. I think I'm right in saying 1952 through 2008 and the researchers found that and I quote them here. Debates are short term events, so they have less effect on people's choices. These performances just get added into this jive and pile of information. And another study looking at American debates through 2020 found the same thing. I think it's really clear that that's, Kamala Harris, has come out, from, as it were, under Joe Biden, the debate name handing over, the sort of, the baton, as it were. She's really ignited the Democratic Party. She had a successful national convention. But at the end of the day, with this being such a short, campaign period, this is really important. So. Could this be the debate, the event, the night that really matters for after all, we know who Donald Trump is. The American people know who Donald Trump is, that they know less about Kamala Harris. I think that's right. I mean, I think first, the studies are right. These are not game changing moments. Usually they're part of a bigger story of how voters are shaping their preferences. They're not irrelevant, but they're not, events that all of a sudden transform everything. We just went through an exception, the Biden, Trump debate was an exception, which fundamentally changed the nature of the contest tonight. you're correct. I think there's less to know about former President Trump. Most voters know exactly who he is, what he sounds like, how he is in political competition and combat. So more of the pressure is on her, again, to show a little bit about who she is as a person. I think more important than the substance of what she stands for, and also to see how she performs. I mean, this is a vision of how will she compete in the political arena as president, facing someone like the former president? Finally, she doesn't want to make a big mistake. I mean, I think a lot of the clips you played revolve around mistakes that people make. Those become a centerpiece rather than the positive. So all of this is on her shoulders when the debate takes place. Julian, she's a trained prosecutor. I mean, a highly successful prosecutor. She's a comfortable public speaker with hold at least. although she's pretty much for most of the American, people who care to have what she's been working off a teleprompter, since, she's, been this, presidential contender. She is significantly younger than Donald Trump. He is, though, of course, a showman and quick on his feet with quips. How do you expect the debate to play out? Ultimately? precision versus chaos, I think if if both are up to their normal strategy, she will be like a prosecutor. She will level attacks on him, dealing with everything from the way in which he has raised threats, to different people in the electorate, to questions about his basic philosophy on issues like reproductive rights. And they will be very, very targeted. Comments. That's how she is in debates when she's at her best. He will be chaos. That is what he likes. He likes to throw everything at his opponent, try to rattle them, try to spread disinformation about them while everyone is watching. and in the end, it will be different. Voters will take different elements of that. I don't expect anything different again, unless one of them, has trouble tonight. once the cameras are rolling, Let's remind ourselves this is such a close race. It's so closely contested, according to the polls, at least at present, that in the end, there will be millions watching this, both in the States and around the world, both on television and on digital and social channels, as this is sort of. And then cut up, of course. and published in chunks or soundbites. But ultimately, let's remind our viewers that there is a very small number of key voters that these two candidates have to go after and win at this point. Correct. We're talking in the tens of thousands here. I think that's right. I mean, debates in the 70s and 80s, when they really got going were about convincing millions of voters to vote one way or another. Now we're just talking about thousands. It's still matters because those thousands will determine the election in the key swing states, for the electoral college. But they're not trying to convince everyone to come to the other side. They're just looking at pockets of the American electorate and asking them either to make sure they come out to vote because of what they see tonight, or even smaller group that they might consider voting for someone other than who they were once thinking of support in.