US president Donald Trump has said he will not accept the blame if his party loses control of the US House of Representatives.

Three weeks before election day in the US mid-term polls, Trump argued that his campaigning and endorsements have helped Republican candidates.

He added that he senses voter enthusiasm rivalling that which propelled him to the White House in 2016, and expressed cautious optimism that his most loyal supporters will vote, even when he is not on the ballot.

Trump dismissed suggestions that he might take responsibility, as his predecessor Barack Obama did, for mid-term losses, or view the outcome as a referendum on his presidency.

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“No, I think I’m helping people,” Trump said. “I don’t believe anybody’s ever had this kind of an impact.”

The president spoke on a range of subjects during the interview, defending Saudi Arabia from growing condemnation over the case of a missing journalist, accusing his lawyer Michael Cohen of lying under oath, and showing defiance when asked about the “Horseface” insult he hurled at Stormy Daniels, the porn actress who accuses him of lying about an affair.

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Stormy Daniels accused Donald Trump of lying about an affair

Asked if it was appropriate to insult a woman's appearance, Trump responded: "You can take it any way you want."

Throughout much of the nearly 40-minute interview, he sat, arms crossed, in the Oval Office behind the Resolute Desk, flanked by top aides including White House press secretary Sarah Huckabee Sanders and communications director Bill Shine. White House counsellor Kellyanne Conway listened from a nearby sofa.

The interview came as Trump's administration is being urged to pressure Saudi Arabia to account for the disappearance of journalist Jamal Khashoggi from the Saudi consulate in Istanbul.

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Trump offered a defence for the US ally, warning against a rush to judgment, similar to what he said happened with his US supreme court nominee, Brett Kavanaugh, who was accused of sexual assault.

"Well, I think we have to find out what happened first," Trump said.

"Here we go again with, you know, you're guilty until proven innocent. I don't like that. We just went through that with Justice Kavanaugh. And he was innocent all the way."

Weeks away from the mid-term elections, Democrats are hopeful about their chances to recapture the House, while Republicans are increasingly confident they can retain control of the US senate.

The National:

Donald Trump speaking in the House of Representatives

Trump has been campaigning aggressively in rallies aimed at invigorating his support base. He said he believes he is doing his job, but acknowledged that some of his supporters say they may not vote this November.

"I'm not running," he said.

"I mean, there are many people that have said to me ... 'I will never ever go and vote in the mid-terms because you're not running and I don't think you like congress.'"

He added: "Well, I do like congress."

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If Democrats take the House and pursue impeachment or investigations – including seeking his long-hidden tax returns – Trump said he will "handle it very well".

The president declared he was unconcerned about other potential threats to his presidency. He accused Cohen of lying when testifying under oath that the president coordinated on a hush-money scheme to buy Daniels' silence.

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Michael Cohen has been accused of lying by the US president

Trump declared the allegation "totally false". But in entering a plea deal with Cohen in August, federal prosecutors signalled that they accepted his recitation of facts and account of what occurred.

The US president said Washington lawyer Pat Cipollone will serve as his next White House counsel and that he hoped to announce a replacement for UN Ambassador Nikki Haley in the next week or two.

He again repeated his frustration with US attorney general Jeff Sessions over the special counsel investigation, saying: "I could fire him whenever I want to fire him, but I haven't said that I was going to."

On the ongoing Russian collusion investigation, Trump defended his son Donald Trump Jr over a Trump Tower meeting with a Kremlin-connected lawyer offering damaging information about Democrat Hillary Clinton.

Trump called his son a "good young guy" and said he did what any political aide would have done.

The US president again cast doubt on climate change, suggesting, incorrectly, that the scientific community was evenly split on the existence of climate change and its causes. There are "scientists on both sides of the issue," Trump insisted.

"But what I'm not willing to do is sacrifice the economic well-being of our country for something that nobody really knows."

He added: "I have a natural instinct for science, and I will say that you have scientists on both sides of the picture."

Repeatedly stressing what he saw as the achievements of his first two years, Trump said he would be seeking another term because there was "always more work to do".

"The new motto is Keep America Great," the US president said.

"I don't want somebody to destroy it because I can do a great job, but the wrong person coming in after me sitting right at this desk can destroy it very quickly if they don't do the right thing. So no, I'm definitely running."