GWEN IFILL: Now looking ahead to the debate, we begin by hearing from both campaigns.
A short time ago, I spoke with Clinton campaign manager Robby Mook and asked what his claims — about his claims that Hillary Clinton is being held to a double standard in advance to tonight's showdown.
ROBBY MOOK, Clinton Campaign Manager: Well, Gwen, we just want both candidates to be graded according to the same criteria.
First of all, both of these candidates need to demonstrate a command of the issues. That's something Hillary Clinton has done. We just haven't seen that yet from Donald Trump. We need to hear about some real plans. Again, Donald Trump has not — still hasn't rolled out, for example, a plan on ISIS.
His tax plan has changed multiple times, has been ridiculed basically every time it's come out. And, lastly, Donald Trump has got to tell the truth. You cannot get a passing grade after telling I would argue even a single lie in this debate, let alone the string of lies that he usually tells.
So, we just want people to grade as if the next president of the United States is on that stage and we're judging them. There is no curve when you get into the Oval Office. There shouldn't be a curve at this debate.
GWEN IFILL: So, you make the case that the bar has been set too low for Donald Trump.
And yet major news outlets this weekend wrote basically long stories saying that he's a liar. So, how can you make that argument?
ROBBY MOOK: Well, we're asking for people to judge the performance in the debate tonight. If Donald Trump doesn't tell one of his normal lies, that's great.
But once we get through that bar, which I would argue is very low, we need to hear a real, true command of the issues. Can he actually speak for 90 seconds — that's the length of the answers — in a substantive way about policies? And does he actually have concrete ideas?
He has a tendency, we saw at the Commander-in-Chief Forum a few weeks ago, to ramble on and repeat himself, repeat things that may or may not be true. That's simply not enough. We're picking our next president. It's time to get serious. And voters expect him to meet a very high bar to become our next president.
GWEN IFILL: I don't expect you to tell me all of your debate prep secrets. But I am curious about how much of the time in preparing Secretary Clinton for this debate you spent talking or thinking about how to get inside Donald Trump's head.
ROBBY MOOK: Well, look, Gwen, as you're pointing out, one of the things that's very troubling to voters about Donald Trump is his erratic behavior, his lack of good temperament to serve as commander in chief.
This is not the kind of personality you want controlling our nuclear codes or commanding our men or women in uniform. So, we don't know how Donald Trump will act at this debate. I would argue that is a threshold reason he's not qualified to be president.
But Hillary Clinton is steady, experienced. She has been on that debate stage before one on one. I'm sure she will be prepared for whatever comes her way.
GWEN IFILL: Secretary Clinton is also on that stage tonight. Which audiences is she aiming to, is she hoping to speak to, aside from criticizing Donald Trump?
ROBBY MOOK: Well, Gwen, I'm really glad you asked that question, because we think that this debate is a really important opportunity for Secretary Clinton and for this campaign.
A lot of voters are going to be checking into this debate for the very first time. Every time Secretary Clinton has had the opportunity to talk about her proactive plans and ideas about how to make people's lives better, she's done better on the campaign.
There are undecided voters still. There are people who are supportive, but maybe not quite as motivated to go vote yet. We see this as an opportunity to speak to all of those voters. And that's the most important thing she can do, make the case about this lifelong mission she's had to fight for kids and families and how she's going to make a difference in people's lives as president.
GWEN IFILL: How worried are you about this latest round of tightening polls?
ROBBY MOOK: You know, Gwen, there are a lot of polls out there. Particularly public polls go up and down.
The thing is, people use a lot of different methodologies. So, we are just staying focused on running like we're 20 points behind. If there is a voter who isn't registered yet or may not have all the information about how to vote, we are going to go and work hard to make sure that they turn out whether we're ahead or behind in the polls.
So, we're executing the same strategy no matter what. And we're going to look past the polls and go right to the voters.
GWEN IFILL: So, this is the first of three one-on-one debates between the presidential debates. How important is the first one? Or are we going to be having this conversation again, saying the next one is the most important?
ROBBY MOOK: Well, I think all of them are an important opportunity. There are some slightly different formats.
But, look, this is the first one. Of course, voters are probably going to be tuning in to this one more than any other. And, again, we think this is such a valuable opportunity for Secretary Clinton to make her proactive case for what she's going to do to help everyday people, why people should be excited about turning out to vote for her.
So, we're going to take advantage of it.
GWEN IFILL: Well, we will all be on the edge of our seats watching it, probably not as closely as you.
Robby Mook, Secretary Clinton's campaign manager, thank you so much for joining us.
ROBBY MOOK: Thank you very much, Gwen.