5 Storylines to Watch During Tonight's Presidential Debate

Trump and Clinton are going to face off tonight.

— -- As millions of voters tune in to watch the first presidential debate of the 2016 campaign, there are several key issues to keep in mind.

Watch the first presidential debate on ABC News and ABCNews.com at 9 p.m. Eastern time. on Monday.

Debate Prep Plays Out Onstage

She has said she expects it to be a "difficult, challenging" debate. Her campaign has even consulted with "The Art of the Deal" ghostwriter Tony Schwartz on how to beat him in a debate.

Trump was in debate prep meetings all day Friday after focusing more on campaigning than studying up for the showdown. Though no mock debates occurred, he was asked rounds of questions he could face from tonight's moderator.

Testing Their Tones

Acknowledging that there are concerns about his tone, Trump has repeatedly said that he plans to be polite — but only to a point.

"I'm going to be very respectful of her," Trump said during a interview on Fox News on Thursday.

"I think she deserves that, and I'm going to be nice. And if she's respectful of me, that’ll be nice. We'll have something that I think people will respect as a debate, but we'll see where it all goes. You really never know exactly how it's going to turn out, and that's why we're going to have a lot of people watching," he said.

For her part, Clinton and her team have spoken about how they had to prep by facing off against two versions of Trump: the more professional, contained version that has been present on the trail more recently and the more bombastic, uncontrolled version that was seen more during the primaries.

Famous Faces in the Audience

There are expected to be about 1,000 people in the audience.

The Role of the Moderator

Lester Holt from NBC's "Nightly News" has been tasked with moderating tonight's debate, and he's under some pressure.

Critics complained that he focused more on Clinton's email scandal than anything to do with the military and that he let Trump make claims that were false without any intervention.

The question of how much fact-checking moderators should do during the debates has been raised. Trump has said that he does not believe the moderators should intervene to fact-check, and Clinton raised money off Lauer's lack of fact-checking after the forum, indicating that she would be in favor of moderators' playing a role.

The Spin Efforts Afterward

It's estimated that tens of millions of people will watch the debate in real time, and many more will be influenced by the social media spin in the aftermath, when the debate about the debate takes place online.

Clinton's campaign is well aware that the outcome of the debate depends not just on what the two candidates say onstage but also on the conversation that takes place around it.

On a conference call with supporters on Friday, a top aide to Clinton directly asked their supporters to use social media during and after the debate to help shape the conversation positively around the Democratic nominee.

The campaign's digital director, Jenna Lowenstein, said two of the campaign's goals for the night are to amplify Clinton's best moments online and to influence the narrative about who is winning.

"It's important that we're not just turning this into the Donald Trump show," Lowenstein said.

She instructed, "Tweet early, tweet her name, use those hashtags."

Watch the first presidential debate Monday at 9 p.m. ET. Full live coverage and analysis of the debate will begin on ABCNews.com/Live at 7 p.m. ET.