NEW YORK (Reuters) - Donald Trump's former fixer Michael Cohen will resume testifying on Tuesday at the Republican presidential candidate's criminal trial, a day after telling jurors that Trump personally authorized him to make a hush money payment to a porn star weeks before the 2016 election.

Cohen, once so loyal to Trump that he claimed he would take a bullet for his boss, is the prosecution's most important witness. In hours of testimony on Monday, he said Trump ordered him to pay the adult film actress Stormy Daniels - "Just do it," Cohen remembered Trump saying - to ensure her silence about an alleged 2006 sexual encounter.

Cohen's $130,000 payment in October 2016 is at the heart of Trump's trial, the first for a former U.S. president, which began in New York state criminal court in Manhattan a month ago.

Prosecutors say Trump paid Cohen back after the election and hid the reimbursements by creating false records indicating they were for legal fees. Those reimbursements provide the basis for the 34 counts of falsifying business records that Trump faces.

Trump, 77, who is running against Democratic President Joe Biden in November, has pleaded not guilty and denies any sexual encounter with Daniels. He has characterized the case as a partisan attempt to interfere with his campaign to take back the White House.

During his first day on the witness stand, Cohen, 57, described multiple episodes in which he said Trump approved payments to keep damaging sex-scandal stories out of the public eye, lest they torpedo his presidential campaign.

"Everything required Mr. Trump's sign-off," Cohen said.

In October 2016, he said, he learned that Daniels was shopping her story to tabloids. At the time, the Trump campaign was in crisis mode after 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 told jurors Trump had said. "'Guys, they think it's cool, but this is going to be a disaster for the campaign.'"

Cohen testified that Trump was solely concerned about the impact Daniels' story could have on his White House bid - and not, as Trump's defense lawyers have suggested, about the effect on his wife and family. That distinction is crucial to the prosecution's case.

Under New York law, falsifying business records can be elevated from a misdemeanor to a felony if the crime helped conceal another offense. In Trump's case, prosecutors have argued that the payment was effectively a secret contribution to his campaign, violating federal and state laws.

Cohen's own past dishonesty - he pleaded guilty to federal crimes in connection with the Daniels payment and has admitted lying under oath multiple times - is sure to prompt a bruising cross examination from Trump's lawyers whenever he concludes his direct testimony.

Defense lawyers have already signaled their intention to attack his credibility, calling him a liar in their opening statement and warning jurors not to trust his word.

'CATCH AND KILL'

Cohen said that he, Trump and tabloid publisher David Pecker had secretly agreed in 2015 to use the National Enquirer to help Trump's campaign.

That arrangement included a $150,000 payment from Pecker's company to former Playboy model Karen McDougal to buy her story about a year-long affair she said she had with Trump, Cohen said. Trump has also denied that relationship.

As in the Daniels instance, the intent was to acquire the rights to the story only to bury it, a practice Pecker called "catch and kill."

Cohen's most dramatic testimony concerned the chaotic final weeks of the 2016 campaign, when he arranged to pay Daniels through a phony shell company to disguise the nature of the payment.

Days before Election Day, the Wall Street Journal published a story about the McDougal hush money deal with Pecker's company that also mentioned Daniels. Cohen testified that he and campaign spokesperson Hope Hicks frantically worked on a statement denying the story, while prosecutors showed jurors emails exchanges between them.

That testimony could undercut any defense claim that the hush money payments were not related to the campaign.

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 considered 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.

(Reporting by Jack Queen, Luc Cohen and Joseph Ax; Editing by Noeleen Walder and Leslie Adler)

By Jack Queen, Luc Cohen and Joseph Ax