Trump's accusing this afternoon's moderator of the US candidate debate of being a Democrat - giving Clinton unfair advantage.

LISTEN ABOVE: US political commentator Dominic Carter spoke to Rachel Smalley ahead of the debate

The showdown between Hillary Clinton and Donald Trump comes six weeks out from election day on November 8.

Fortune magazine's tipped it to be the most-watched presidential debate in history, and as popular as the Superbowl.

US political commentator Dominic Carter told Rachel Smalley it will be moderated by NBC's Lester Holt, who's a known Republican.

"Trump is screaming a damn bloody murder, Rachel, that he's really a Democrat. So everything is up in the air and this great show 100 million people, the theatre if you will, begins in just a few hours."

He said many are scratching their heads as to how Trump's come so far, but his TV experience gives him an edge in the debate.

Carter said Trump's character has tapped into American discontent, he's good on his feet and he connects - all things Bill Clinton has, but Hillary does not.

University of Auckland Professor Stephen Hoadley said while millions of eyeballs will be on the 90-minute debate, it's likely to be more about entertainment than policy.

Both candidates will be out to undermine each other, he said.

"The factual content again won't be very high. But the supporters of each will look for reinforcement of their preconceived notions. I don't think the debate's going to change anyone's vote."

He said Donald Trump is likely to play up his freewheeling rhetoric, as someone who's in touch with the ordinary American.

Trump will try to present as one who can make quick decisions, doesn't waffle and isn't intellectual, but is a "true American" - and it's this that Clinton will have to undermine by tripping him up, said Hoadley.

As to the implications for New Zealand in the case of a Clinton versus Trump Presidency, it's a little harder to predict, he said.

While Clinton is more likely to continue on the same path as President Obama, Trump's foreign policy is a bit more unique.

"Make the Allies pay their way and stop depending on Uncle Sam for protection, aid and other benefits. So, it could well be that a Trump presidency would reduce exercises and contact with New Zealand."

Although the entertainment factor this debate is really the one that counts, says CNN's senior media correspondent Brian Stelter.

"There'll be two debates after this, three in total but the first one is traditionally the highest rated. A lot of people who are political junkies have been paying attention for over a year but now is the moment the rest of the country, and the rest of the world, tunes in for the first time."