When Donald Trump takes to the debate stage for the second time this year, this time against Kamala Harris, he will be facing questions about whether he is too old or mentally unfit for a second term as president.

His former rival, President Joe Biden, was ultimately removed from the Democratic Party ticket following his stuttering performance in a June debate with Trump, and Vice President Harris is now the younger and more energetic candidate.

The debate, hosted by American broadcaster ABC and held in the key swing state of Pennsylvania, will be held live at 11am AEST on Wednesday, September 11.

Nine, Seven, ABC and SBS will broadcast it in Australia on free-to-air TV.

The issues

Trump has made immigration a key cornerstone of his campaign, promising the mass export of non-citizens in what he said will be a “bloody story”.

“[They] should have never been allowed to come into our country,” he said.

“Nobody checked them.”

Trump and Vance have made immigration and border security a key part of their campaign. Photo: Getty

The Harris campaign has instead pointed to major investments in infrastructure and job creation as successes of the Biden administration, while also promising tax credits for newborns and bans on price gouging if elected president.

“My plan will include new penalties for opportunistic companies that exploit crises and break the rules, and we will support smaller food businesses that are trying to play by the rules and get ahead,” Harris said earlier in the month.

“We will help the food industry become more competitive because I believe competition is the lifeblood of our economy.”

The biggest narrative in the election, however, is the prosecutor versus the felon.

Although Trump has a lengthy list of charges and indictments he has only faced one trial, which resulted in 34 felony convictions.

This is due to delays over presidential immunity and appeals.

He won’t receive his sentence before the election, but Harris responded to supporters chanting “lock him up” with encouragement to let the courts do that, and that their job is to beat him at the ballot on November 5.

There has also been an increased focus on Trump’s ability to rise to the rigours of a second presidential term, particularly after a garbled response to a question about child care left people scratching their heads.

The rules

The debate rules, set before Harris was the nominee, dictate that the 90-minute broadcast will include two ad breaks, no props or pre-written notes and the debater’s microphones will be muted when they aren’t answering a question.

There won’t be a live audience, aside from reporters and journalists covering the event.

In an interview with Fox News on Wednesday, Trump attacked ABC and the moderator of the debate.

“They are the most dishonest network,” Trump said.

“They’re very nasty. I think a lot of people are going to be watching to see how nasty they are.”

The playing field

Most aggregators have Harris with about a 3.5 per cent lead on Trump in national polls, but what really matters is the key swing states that will decide the election.

Polling in Wisconsin, Michigan, Pennsylvania, Nevada and Arizona have the two neck and neck, while Trump is enjoying a small lead in Georgia, where Biden beat Trump by less than 12,000 votes.

Harris and her running mate Tim Walz will tour several battleground states after the debate.

With less than 60 days until the election, Harris will kick off the tour in North Carolina on Thursday and travel to Pennsylvania on Friday.

Walz will travel to Michigan and Wisconsin.

Harris’ candidacy has re-energised Democrats and donors, and she has had a surge in opinion polls.

Her campaign raised $US361 million ($541 million) in August, leaving her with a clear cash advantage over Trump.

-with AAP