The first presidential debate tonight is shaping up to be one of the most-watched political events ever, with a potentially Super Bowl-size audience.
Here are four things to watch for as Hillary Clinton and Donald Trump take the stage at Hofstra University on Long Island.
1. Which Trump shows up
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Donald Trump "won" the primary debates by dominating his opponents, often by name-calling and bluster. This one will be different.
Instead of multiple opponents he will be doing something he's never done before — face off against just one (and this case an experienced one) opponent on a debate stage.
Trump's goal is to present himself as a plausible president, someone voters can imagine as commander-in-chief. And he has work to do since majorities of voters say he doesn't have the judgement or qualifications to be president. So Trump needs to show a basic command of policy — and in particular, his own policies.
He's made so many contradictory statements about his plans for Syria, ISIS, tax reform and crime fighting that he will have a real thicket to untangle.
Trump wants to reach those voters who won't want to vote for Clinton but are worried about his temperament. So does that mean the new "teleprompter Trump" will show up tonight? He's proven that he can maintain a little more discipline in a set-piece speech, but there are no teleprompters in a 90-minute debate.
And although Trump benefits from low expectations, his team isn't even trying to make him out to be the underdog. On Sunday morning, Trump's campaign manager Kellyanne Conway called him "the Babe Ruth of debating."
2. Whether Clinton can navigate the gender minefield
Clinton has the much tougher task tonight. She has the burden of high expectations.
The former senator and secretary of state, who's now been through two presidential campaigns, is an experienced debater who knows policy inside and out.
But her job is very hard — Clinton has to convince voters who don't want to vote for Trump but haven't warmed up to her that she is likeable, honest and trustworthy. And she has to press her case that Trump is unqualified to be president without being overly aggressive or "harsh."
Gender communications research shows that debaters who are on offense win, and those that are on defense lose. But being on offense for a woman is tricky. Male debaters who are aggressive are perceived positively, female politicians who are aggressive in debates are perceived negatively.
So Clinton has to stay on offense without being angry. All that advice about "smiling" that drives Clinton's supporters nuts? It's unfair, but that's just the way it is says Brett O'Donnell, a veteran Republican debate coach: "People like to see a happy warrior. Clinton has to look like she's enjoying herself even if she's not."
3. The moderator
Both campaigns have been working the refs hard. Trump has said the debate system is "rigged" against him and falsely accused NBC's Lester Holt, the moderator for tonight's debate, of being a registered Democrat. Holt is an experienced journalist who happens to be a registered Republican.
The Clinton campaign, on the other hand, has complained about a double-standard. They say the media creates a "false equivalence" between Trump's falsehoods and Clinton's, even though numerous fact checks have shown that Trump prevaricates many more times than Clinton does.
In the recent NBC commander-in-chief forum, Clinton's top aides said moderator Matt Lauer grilled Clinton a lot more intensely than Trump. And they say that for tonight's debate the moderator and the TV networks — with the crawl at the bottom of the screen — have a responsibility to fact-check Trump in real time.
4. Which campaign better argues they "won"
There are three phases to a debate:
First, the pre-game expectations setting and referee massaging, which has been going on at a furious pace over the past week.
Second, the debate itself.
And third, the post-debate spin.
Debates are not forums to score policy points. They are tests of character and demeanor. And they are often judged not in their totality, but by "moments" — the zingers and put downs that the candidates prepare in advance. Those moments — "Where's the beef?" "You're no Jack Kennedy," "There you go again" — help determine who voters think won or lost the debate.
But there are many debates that were "won" in the hours after the candidates left the stage by the campaign that was more adept at getting their narrative into the media.
Follow along with the NPR Politics team's coverage of tonight's debate, which starts at 9 p.m. ET, at npr.org and on many NPR member stations.