(CNN) Donald Trump will have 90 minutes Sunday night to save his presidential campaign, as he faces off against Hillary Clinton in a debate that will cap one of the most extraordinary weekends in American political history.

Republicans -- including vice presidential nominee Mike Pence -- are criticizing Trump for his vulgar comments about advances he has made toward women that came to light Friday. And an ever-growing list of senators and top GOP officials want Trump replaced on the ticket.

Trump insists he won't leave the race, and he and allies (those that remain, at least) indicate he'll go on the attack against Clinton.

There's one thing on everyone's mind Sunday night in St. Louis at the town hall debate co-moderated by CNN's Anderson Cooper. Here's what to watch:

The tape -- what will Trump say?

Soon after the "Access Hollywood" tape became public on Friday, Trump released a statement that read, in part: "I apologize if anyone was offended."

That wasn't sufficient. Hours later, Trump released a video message: "Anyone who knows me knows these words don't reflect who I am. I said it, I was wrong, and I apologize," Trump said, looking into the camera.

He made a brief appearance Saturday on Fifth Avenue in front of Trump Tower, and a couple of calls to newspapers vowing to stay in the race. But this will be his first live appearance and one that guarantees follow-up questions.

Although Trump has apologized, he has not yet addressed serious concerns raised by the hot mic video. Most notably, Trump's suggestion that he made aggressive advances toward women -- including the comment that he would "grab them by the pussy" -- has raised grave new questions about whether he touched women without their consent.

This is Trump's chance to convince Americans that he is sincere in his apology and that he can be trusted as commander in chief. If he can do that, as well as score points on his core issues of trade and security, he may be able to survive.

What will Trump say about Bill Clinton?

After the first debate, Trump congratulated himself for refraining from bringing up Bill Clinton's "indiscretions."

"I'm really happy I was able to hold back on the indiscretions in respect to Bill Clinton. Because I have a lot of respect for Chelsea Clinton," he told CNN's Dana Bash. When pressed by Bash on what he would have said, Trump responded: "Maybe I'll tell you at the next debate."

It now appears that Trump may do exactly that on Sunday.

The GOP nominee has already given two indications that he is preparing to target the former president -- and the Clintons' marriage -- on the debate stage.

Immediately after the "Access Hollywood" video was released, Trump said in a statement: "Bill Clinton has said far worse to me on the golf course - not even close."

In the video message hours later, Trump capped his curt apology with this: "Bill Clinton has actually abused women and Hillary Clinton has bullied, attacked, shamed and intimated his victims."

"We will discuss this more in the coming days," Trump added.

And he retweeted two tweets from Juanita Broaddrick, the Arkansas nursing home administrator who alleged that Bill Clinton had raped her in 1978. Clinton has denied the allegations, and there was never any legal action against him.

The strategy of dredging up Clinton's extramarital affairs, such as those with Monica Lewinsky and Gennifer Flowers, enjoys minimal support within the Republican Party , and top GOP leaders are wary of going down this path. Given the timing, it could also be interpreted and dismissed as desperate.

What does Hillary Clinton say?

Clinton has kept a low profile all weekend, staying in New York for debate prep, and so this will be the first time she responds in person to the Trump tape.

In recent weeks, the Democratic nominee had already adopted highlighting Trump's past remarks about women as a major strategy, aimed at hammering home the point that Trump is temperamentally unfit to be president. The Clinton campaign has featured Alicia Machado, for example -- a former Miss Universe whom Trump has made disparaging comments about -- in online ads, and facilitated Machado sharing her story with the media.

Clinton may be less keen on addressing her husband's extramarital affairs if Trump decides to go there.

Asked last month whether she feels any obligation to object to a spouse's indiscretions being brought up in the campaign, Clinton simply answered: "No."

Clinton faces questions on her speeches

The Trump bombshell Friday came at roughly the same time the Clinton team was dealing with problems of its own -- WikiLeaks posted thousands of hacked emails from Clinton campaign chairman, John Podesta, that appear to include excerpts of Clinton's private speeches to Wall Street companies.

Those excerpts could validate what some of Clinton's critics have said all along: that the Democratic nominee is out of touch with the middle class.

In one 2014 speech, Clinton appears to acknowledge that she is "kind of far removed" from the struggles of everyday Americans -- comments that could haunt her in Sunday's debate and on the campaign trail.

"My father loved to complain about big business and big government, but we had a solid middle class upbringing," she said in the remarks. "And now, obviously, I'm kind of far removed because the life I've lived and the economic, you know, fortunes that my husband and I now enjoy, but I haven't forgotten it."

With all the attention on Trump, the excerpts haven't gotten much attention. But figure Clinton will have to explain her reasoning for those comments Sunday.

Trump's first real town hall

Throughout the 2016 campaign, Trump has largely stuck to a campaign method that he's most comfortable with: large rallies.

Even during the primary season, when presidential candidates typically prioritize retail politics and small gatherings that allow maximum hand-shaking and baby-holding, Trump mostly steered clear of intimate campaign settings.

That means that the GOP nominee will head into Sunday night a novice in town halls -- a format that will expose the candidates to questions from audience members and require extra agility and grace. His lack of practice with the format could hand him a significant disadvantage.

Even at an event that the Trump campaign billed as a "town hall" in New Hampshire Thursday, the format was such that the nominee didn't interact with audience members (the questions were read out loud, instead, by moderator Howie Carr), and answered softball questions for just over 30 minutes.

The prime-time event will be co-hosted by Cooper and ABC's Martha Raddatz, but the real wild card is the audience members who will grill Clinton and Trump.

Past town hall debates have produced famously awkward moments, including John McCain referring to President Barack Obama as "that one," and George W. Bush prompting the audience to bust into laughter with a raise of the eyebrows and a head nod at ex-Vice President Al Gore.

The debate also has the potential to produce some raw interactions between voters and the candidates.

At a town hall-style event in Haverford, Pennsylvania, last week, a woman told Clinton that she had lost two sons -- one died from gun violence and another had committed suicide, she said.

As a campaign volunteer hugged the woman, Clinton responded: "Thank you for being so brave."