Hope Hicks, a former top aide to Donald Trump, has testified that he told her in the final days of the 2016 presidential election to deny that he had a sexual relationship with porn star Stormy Daniels.

Hicks’ testimony gave jurors an inside look at the campaign’s damage-control efforts when Trump faced multiple accusations of unflattering sexual behaviour in the waning weeks of his successful White House campaign.

Trump has pleaded not guilty to charges of falsifying business records to cover up a $US130,000 ($196,600) payment made at that time to Daniels, who was threatening to go public with her story of their 2006 sexual encounter.

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Hicks testified that she told Trump four days before the November 8, 2016 election that the Wall Street Journal would publish details of Daniels’ story.

“He wanted to make sure that there was a denial of any kind of relationship,” said Hicks, who served as campaign press secretary.

She said Trump did not want his wife Melania to see the story, which also included allegations that he had an affair with former Playboy model Karen McDougal.

Trump has denied having sex with either woman.

“He was concerned about how it would be viewed by his wife, and he wanted me to make sure the newspapers weren’t delivered to the residence that morning,” Hicks said.

Prosecutors in the first criminal trial of a former US president argue that the payment to Daniels corrupted the election by suppressing news that could have influenced voters as they decided whether to back the Republican Trump or Hillary Clinton, then his Democratic rival.

Prosecutors say Trump falsified records to cover up election-law and tax-law violations, which elevates the 34 counts he faces from a misdemeanour to a felony punishable by up to four years in prison.

Hicks’ testimony could help Trump’s lawyers make their case that he paid off Daniels to keep his wife, not voters, from hearing the adult film star’s allegations.

She said Trump told her that his personal lawyer, Michael Cohen, paid off Daniels to “protect him from a false allegation” out of the “kindness of his own heart”.

Hicks said she thought that would have been out of character for Cohen.

“I didn’t know Michael to be an especially charitable person or selfless person,” she said.

Hicks teared up briefly during her testimony.

Trump, the Republican candidate for president again this year, sat expressionless at the defendants’ table during the testimony by Hicks, the first person who worked for Trump’s campaign to appear as a witness in the 11-day-old trial.

Hicks said the campaign was rattled by the public release of an audio recording from the Access Hollywood TV show in which Trump bragged about grabbing women’s genitals.

She said Trump was upset but also played down the comments.

“Mr Trump felt like this wasn’t good but it was also just like two guys talking, locker-room talk,” she testified.

The 12 jurors and six alternates have yet to hear from the main players in the case, including Daniels and Cohen.

Earlier in the day, the judge overseeing the trial told Trump that a gag order that bars him from commenting about witnesses and jurors would not prevent him from testifying, as Trump had told reporters on Thursday.

“I want to stress to Mr Trump: you have an absolute right to testify at trial,” Justice Juan Merchan said.

Trump said his legal team would try to overturn the gag order, which bars him from making public comments about jurors, witnesses and families of the judge and prosecutors if those statements are meant to interfere with the case.

Merchan fined Trump $US9000 on Tuesday for violating the order and signalled on Thursday he may impose more fines for what prosecutors say are further violations.

Merchan has said Trump could be jailed if he does not change his ways.

Trump says the case is an attempt by Democrats to undercut his chances of defeating Democratic President Joe Biden in the coming November 5 presidential election.

—AAP