Tonight marks the third of four debates, and the second of three featuring the presidential candidates. Given calls for GOP nominee Donald Trump to exit the race by members of his own party, Sunday's debate has taken on a new meaning, and one that will go down in history.
The debate is scheduled to begin at 9 p.m. Eastern, and will last 90 minutes.
Here is what you need to know about Sunday’s showdown.
Democratic Party nominee Hillary Clinton and Trump were the only two candidates invited to Sunday’s debate based on polling. In order to be invited, a candidate must poll at 15 percent or above in a series of national polls to qualify.
The Commission on Presidential Debates picked CNN’s Anderson Cooper and ABC News’ Martha Raddatz as co-moderators for Sunday’s debate. Raddatz was pegged in 2012 to moderate the vice presidential debate between Joe Biden and Paul Ryan. Cooper has not moderated a general election debate.
Raddatz moderated a Democratic Party debate and Republican Party debate during the 2016 primary cycle. Cooper hosted the final Republican Party debate of 2016, and hosted two Democratic Party debates. Cooper also hosted what was described as the “Final Five” forum with the last two Democratic candidates, and three GOP hopefuls.
Sunday’s debate is the only town-hall style debate of the campaign. Some of the questions will be posed to the candidates by members of the audience, which is made up of undecided voters. The moderators will also pose questions based on public interest and trending on social media.
Candidates will be given two minutes to answer the question, with a one-minute rebuttal.
Last time out
Clinton was widely considered the winner of the last presidential debate on Sept. 26 based on scientific polling. In a poll conducted by CNN, 62 percent claimed Clinton was the clear winner, compared to 27 percent for Trump.
Election polling following the Sept. 26 debate has dramatically changed the dynamic of the 2016 election. The statistical analysis site 538 showed that Clinton had a 54 percent chance of winning the election on the day of the debate, and Trump was quickly gaining on Clinton. Within 10 days, Clinton’s odds jumped to 79 percent. Clinton went from falling behind in states such as Florida, Nevada and North Carolina to slightly ahead. She also has gained ground in Ohio, Iowa and Arizona.
Since the last debate
Perhaps more damning to Trump’s campaign than his lackluster debate performance was a trio of controversies in the days that followed, which has left a number of prominent GOP leaders abandoning Trump's campaign.
The controversy that has done the most damage was the release of a private audio recording from 2005 by Access Hollywood. The recording contained a number of crude comments from Trump where he suggests he can get away with sexually aggressive behavior due to his celebrity.
"I’m automatically attracted to beautiful ( women), I just start kissing them. It’s like a magnet," Trump said.
Just kiss. I don’t even wait. And when you’re a star, they let you do it. You can do anything. Grab ’em by the p----. You can do anything."
Given that 16 Republican senators have since denounced Trump's candidacy, with some calling for him to leave the race, Trump will unquestionably be obliged to answer questions on this issue.
The Access Hollywood tape is not the only issue Trump has to deal with tonight.
After Clinton claimed during the first debate that former Miss Universe winner Alicia Machado was humiliated by Trump, Trump responded on Twitter, calling Machado “disgusting” and a “con.”
And then last weekend, the New York Times uncovered Trump’s tax return from 1995 showing the businessman lost more than $900 million. The tax return showed Trump not paying any federal taxes that year, and also that Trump could have avoided federal taxes in subsequent years.
Unlike other presidential candidates in the last few election cycles, Trump has not offered to release his tax returns, citing a federal audit. The IRS claims Trump is not obligated to withhold his tax documents from public dissemination, however.
One small bit of good news for Trump’s campaign came from his running mate’s performance in Monday’s vice presidential debate. CNN polling showed that Gov. Mike Pence was considered the winner of Monday’s debate over Sen. Tim Kaine by a 48-42 margin. One note though is that Monday’s vice presidential debate was the least viewed debate since 2000.
For a candidate who is falling behind the polls, Sunday may give very little opportunity for Trump to address policy, rather than the controversies built up in the last two weeks.
Questions left unanswered
One topic not broached at the first presidential debate was healthcare. Earlier this week, Bill Clinton called the Affordable Care Act the “craziest thing in the world” while campaigning for his wife. Given Hillary Clinton’s support of keeping Obamacare, and the fact healthcare was not brought up in the last debate, expect Clinton and Trump to be pressed on the issue by Cooper and Raddatz.
Other huge topics left off the table on Sept. 26 were immigration, border security and accepting Syrian refugees. Clinton and Trump differ greatly on these issues as Trump has called for tighter restrictions on entering the United States, even for those looking to flee war-torn Syria. Clinton has opposed completing a wall between the United States and Mexico, and has said the US should welcome Syrian refugees.
Questions worth asking again
Since Trump’s 1995 tax return is up for debate, so are the candidates’ tax plans. While Trump has pushed for an across-the-board tax cut, he has not said how he will pay for the cuts. Based on analysis from the nonpartisan Brookings Institute, Trump’s tax plan could add up to $10 trillion to the United States debt over the next 10 years. Meanwhile, Clinton has not specified her stance on the tax rate for the middle class, only stating a desire to raise taxes on the wealthiest 1 percent.
All about Bill
While Bill Clinton’s comments on Obamacare is likely to come up, so might the former president’s past scandals. Trump has openly weighed bringing up Clinton’s past extramarital sexual encounters during a debate, but said he did not so during the first presidential debate out of respect for the Clintons’ daughter Chelsea.
At a town-hall forum in New Hampshire on Thursday, Trump didn’t promise to bring it up, rather said he would “see what happens.” And Trump surrogate Rudy Giuliani said on CNN's "State of the Union" today that Trump is considering bringing in Clinton's infidelities up during tonight's debate.
Note: This story has been updated to reflect the last campaign news.