This year’s presidential debates should go down as the most childish and vicious in history – and experts have warned the devolution of US politics could follow.

The three debates leading up to next month’s US election have featured a level of personal attacks, interjections and interruptions between Republican candidate Donald Trump and Democrat Hillary Clinton unseen in any other presidential campaign.

But according to James Cahill, deputy editor of Election Watch at the University of Melbourne, it was Mr Trump’s poor gamesmanship that led to the debates’ downfall.

“I thought Trump, particularly in the first two debates, used what I would regard as the ‘simian alpha male behaviour’ of speaking over Mrs Clinton and frankly the moderators too,” Mr Cahill said.

Mr Cahill said Mr Trump’s use of “stunt guests” to target the Clinton family – more so Bill than the actual nominee Hillary – and to claim he will not accept the election result was “downright off-putting”.

‘Trump even lies about his lies’

While some experts excuse Mr Trump’s poor debate behaviour as the result of inexperience on the podium, his “blatant lies” were a step too far, according to Mr Cahill.

“Politicians lie in a certain way, they favour certain facts over others; they try to change the conversation; they try to change the framing or context of a conversation; they minimise facts that are less desirable to their positions,” he said.

“But to actually lie about known facts and things that are verifiably untrue, that is a level of lying we haven’t seen before.”

A compilation of  presidential debate low points below:

“He [Mr Trump] will lie that what he said before wasn’t a lie and he’ll just keep doubling down on it, on things that are verifiably untrue.”

Mr Cahill compared the Republican presidential candidate to a child learning how to lie – but one that never learnt how to get away with it.

“Donald Trump’s lies always remain at the level of what I expect from a five year old.”

State of US politics

US politics could change forever if Donald Trump wins. Photo: Getty

A devolution of the American political system is a genuine fear for some experts and is largely dependent on the outcome of the election.

If Mr Trump loses by a blow-out margin, it is much less likely anyone will try his style of campaigning again.

But if he wins, US politics could become “Trumpatised”, which could be only the beginning of the Republican Party’s problems.

“If Trump wins there is going to be way bigger problems than just the coarsening of a public debate,” Mr Cahill said.

“Trump winning will be a massive upending of the political system that will likely lead to the rupturing of the Republican party.

“Most of the party leadership is against him and I believe they secretly don’t want him to win, I think they know it would be a disaster.”