Donald Trump's former fixer Michael Cohen has told jurors that the Republican presidential candidate personally approved a hush money payment to bury a porn star's story of a sexual encounter before it could derail his 2016 campaign.
"Just do it," Cohen said Trump told him, instructing him to figure out the best way of paying adult film actress Stormy Daniels $US130,000 ($A196,000) to stay quiet about an alleged 2006 liaison, which he denies.
The October 2016 payment is at the centre of the historic trial, which entered its fifth week in New York state criminal court in Manhattan. Prosecutors have said they could rest their case this week.
In hours of dramatic testimony on Monday, Cohen, 57, once one of Trump's most loyal lieutenants and now the prosecution's star witness, described multiple episodes in which Trump signed off on payments aimed at quashing sex-scandal stories while he campaigned for the highest office in the land.
In the final weeks before the 2016 election, Cohen learned that Daniels was shopping her story to tabloids. It was a pivotal moment for the Trump campaign, which was reeling from the release of an audio recording from the TV show Access Hollywood in which Trump bragged about grabbing women's genitals.
"He said to me, 'This is a disaster, a total disaster. Women are going to hate me,' Cohen, wearing a dark suit and pink tie, testified Trump had said.
"'Guys, they think it's cool, but this is going to be a disaster for the campaign.'"
Prosecutors have said Trump paid Cohen back after the election and hid the reimbursements by recording them falsely as legal retainer fees in Trump's real estate company's records.
Trump faces 34 counts of falsifying business records tied to the reimbursements. Prosecutors say the altered records covered up election-law and tax-law violations - since the money was essentially an unreported contribution to Trump's campaign - that elevate the crimes from misdemeanours to felonies punishable by up to four years in prison.
Trump, who is running against Democratic President Joe Biden in November, has pleaded not guilty and argues the case is a politically motivated attempt to interfere with his campaign to take back the White House.
Trump's defence has suggested the payment to Daniels, who testified last week, was meant to protect his family from embarrassment. But Cohen testified that Trump was solely concerned with the effect on his campaign.
"He wasn't thinking about Melania. This was all about the campaign," Cohen said, referring to Trump's wife. At the defense table, Trump, 77, shook his head.
Cohen also told the 12 jurors and six alternates that Trump urged him to delay sending payment to Daniels' lawyer until after the election, telling him that the story would no longer matter.
Trump's lawyers have argued that Cohen, a felon and admitted perjurer, is lying about Trump's involvement and acted on his own. But Cohen said he would never have taken such drastic steps without Trump's approval.
"Everything required Mr. Trump's sign-off," Cohen said.
Cohen testified earlier in the day that Trump approved other payments to forestall damaging stories.
When Trump was preparing to announce his 2016 campaign, Cohen said, Trump warned him there would be "a lot of women coming forward".
Cohen said he, Trump and National Enquirer publisher David Pecker agreed to use the tabloid to boost Trump's presidential candidacy while blocking any negative stories.
That arrangement included a $US150,000 ($A227,000) payment from Pecker's company to former Playboy model Karen McDougal to buy her story about a year-long affair she said she and Trump had, Cohen said. Trump has also denied that relationship.
Cohen pleaded guilty in 2018 to violating federal campaign finance law by paying off Daniels and testified that Trump directed him to do so. Federal prosecutors did not charge Trump with any crime.
The Manhattan trial is widely seen as less consequential than three other criminal prosecutions Trump faces, all of which are mired in delays.
The other cases charge Trump with trying to overturn his 2020 presidential defeat and mishandling classified documents after leaving office. Trump pleaded not guilty to all three.
Cohen will resume testifying on Tuesday.
