With all eyes on them, former president Bill Clinton shook hands with Melania Trump moments before their spouses took the stage. 

Image source, Reuters

Tonight’s encounter will be a town hall-style debate where the candidates won't be standing behind a podium but sitting on stools. This means body language will be more important than ever. The two will take questions from undecided voters. Nearly three million people have either submitted questions or votes. Clinton will get the first question in the forum, which will be co-hosted by CNN’s Anderson Cooper and ABC News’ Martha Raddatz. Clinton won the coin toss, which is why she’ll field the first question.

Image source, AP

While much of America has reacted to Trump's latest surprise with shock, many of his hardcore supporters are still cheering him on.  

Trump may have just praised Clinton sex-assault accuser Paula Jones as "very courageous" during an astonishing panel before tonight's debate, but in 1999 he branded her "a loser". 

Amid suggestions that Bill Clinton's sexual assault accusers will be in the audience tonight, here's who Hillary Clinton has invited:

• Joe Nudi, a car-worker from Ohio who benefited from the auto-industry bailout
• Eugene Hubbard, a Vietnam veteran and janitor who has been campaigning for Clinton in St Louis
• Martha Soltani, a mother from North Carolina who benefited from a children's health programme that Clinton helped create 
• Ernie Greene, one of the Little Rock Nine, a group of the first African-American students to integrate a high school in Arkansas' capital city
• Janelle Turner, a breast cancer patient who has already been invited to Clinton's inauguration if she wins the election. She recently featured in the campaign ad below:

The first debate between Trump and Clinton made history as 84 million people tuned in.

It broke a record that went back 36 years to Jimmy Carter and Ronald Reagan's 1980 head-to-head, which drew 80.6 million viewers.

Could tonight's debate break the record yet again? Media research firm Deep Root Analytics estimate, external that as many as 89 million viewers could watch this duel.

Anthony Zurcher
BBC North America reporter

"If there were any doubt about what exactly is in store in tonight’s presidential debate, Donald Trump’s impromptu press conference has settled it. We are hurtling toward a political Gotterdammerung - and fast.

For the past 48 hours, Republican leaders have been advising Trump to express contrition and move on from the chaos following his taped boasts of unwanted sexual advances. They see trying to drag Hillary Clinton down with him as high risk, low reward. While Trump’s presidential hopes are likely done either way, he threatens to cause lasting damage to the Republican brand and endanger the party’s down-ballot candidates.

Trump, it seems, doesn’t care. But then he’s never cared about the priorities of the establishment he ran against, so this should come as little surprise. Trump is going to leave no stone unhurled.

Does running with unproven allegations directed at the spouse of a presidential candidate violate every norm of US politics? Yes. Does it turn US politics into tawdry daytime television? Yes.

But Trump fashions himself a counter-puncher who answers every blow with overwhelming force. He’s on the ropes now, trying to land that one decisive haymaker before he hits the canvas." 

Image source, Getty Images

The BBC's Reality Check team is once again poised to dive into the murky undergrowth of the candidates' statements to seek the truth. And judging from the build-up to tonight's encounter it could be very murky indeed. We'll deliver our Reality Check verdicts here as the debate unfolds.

Billy Bush, the TV host recorded with Trump on the now-infamous 2005 sex-boast tape, has just been suspended by NBC.  

See our earlier post. 

Image source, AP

Clinton's director of communications, Jennifer Palmieri, released the following statement on "Trump's latest act of desperation":

"We're not surprised to see Donald Trump continue his destructive race to the bottom. Hillary Clinton understands the opportunity in this town hall is to talk to voters on stage and in the audience about the issues that matter to them, and this stunt doesn’t change that. If Donald Trump doesn’t see that, that’s his loss. As always, she’s prepared to handle whatever Donald Trump throws her way."

Image source, Getty Images

Four years ago during the second presidential debate, then-Republican nominee Mitt Romney was lambasted for referring to the "binders full of women", his reference to potential female candidates for his cabinet while serving as Governor of Massachusetts. 

Romney was answering a question about equal pay when he made that ill-fated remark. The phrase set social media ablaze. 

How quaint that remark seems now compared with Trump's comments on the 2005 'hot mic' tape that has been bending the fabric of the internet all weekend. 

During Trump's Facebook live press conference moments ago, a former Romney adviser tweeted this:

Image source, AP

Given the astonishing Facebook Live that Trump just did with Bill Clinton's sexual assault accusers, there can now be no doubt that the Republican candidate, politically wounded and at bay, will go for the jugular tonight by dredging up these past allegations. He has previously accused Hillary Clinton of "enabling" her husband and trashing his accusers. 

Here's who Trump just appeared with, and a handy primer on their allegations:

Juanita Broaddrick accuses Bill Clinton of raping her in a hotel room in 1978. The nursing home administrator spoke out in 1999 after Clinton's impeachment trial. The former president has previously denied the allegation through a lawyer and no charges were ever filed. Ms Broaddrick also claims Hillary Clinton thanked her for her silence at a political rally.

Paula Jones, a former Arkansas state employee, filed a sexual harassment lawsuit against Clinton in 1994. She alleged the then-Governor of Arkansas propositioned her and exposed himself in 1991. Bill Clinton settled the lawsuit for $850,000 with no admission of guilt.

Kathleen Willey, who was a White House aide, said Bill Clinton groped her in his office in 1993 on the same day that her husband died in an apparent suicide. The former president has denied this allegation, too.

Kathy Shelton was the child victim in a sexual assault case Hillary Clinton defended during her legal career. She was the attorney for a factory worker accused of raping Ms Shelton, who was then 12 years old. In audio recordings from the mid-1980s of an interview Clinton had with an Arkansas reporter, the current Democratic nominee is heard laughing while discussing the trial, including this instance when she says: "Of course he [the defendant] claimed he didn’t [rape her]… He took a lie detector test. I had him take a polygraph, which he passed, which forever destroyed my faith in polygraphs." Ms Shelton's attacker ultimately admitted a reduced charge, and she has since said Clinton put "me through hell".

Trump has just done a Facebook Live with Juanita Broaddrick, Paula Jones and Kathleen Willey, women who have accused Bill Clinton of sexual assault.  

Image source, Getty Images

The TV host who was recorded with Trump on the now-infamous 2005 sex-boast tape is taking a leave of absence.

Billy Bush, who is first cousin to President George W Bush, will not appear on the sofa as co-host of NBC's Today Show on Monday, the network has confirmed.

Bush, 44, has already released a statement saying he was "embarrassed and ashamed".

He's been excoriated on his Facebook page since the tape surfaced on Friday. "You are sickening!" one woman wrote, in a comment that attracted thousands of likes.