NEW YORK (Reuters) - Donald Trump's estranged former fixer Michael Cohen is expected to begin giving testimony on Monday that could determine whether jurors convict the former U.S. president of illegally hiding a payment to silence a porn star who said they had a sexual encounter.

For nearly a decade Cohen, 57, worked as an executive and lawyer at Trump's New York-based family real estate company and once said he would take a bullet for Trump, a Republican trying to take back the White House from Democratic President Joe Biden in this year's Nov. 5 U.S. election.

Trump's personal lawyer from the start of the White House years in 2017, Cohen broke with him when federal prosecutors probing Trump's 2016 presidential campaign honed in on Cohen, now one of Trump's most outspoken critics, frequently disparaging him on social media and on podcasts.

On Friday, Justice Juan Merchan urged prosecutors to tell Cohen to stop making public statements about the case after defense lawyer Todd Blanche said Cohen had spoken on social media while wearing a T-shirt showing Trump behind bars.

Cohen's $130,000 hush money payment to the porn star Stormy Daniels before the 2016 election about a 2006 sexual encounter she says she and Trump had is at the center of the trial, which began on April 15 in New York state criminal court in Manhattan.

Prosecutors with Manhattan District Attorney Alvin Bragg's office say Trump falsely labeled his reimbursement payments to Cohen in 2017 as legal expenses in his New York-based real estate company's books. 

They say the altered business records covered up election-law and tax-law violations that elevate the 34 counts Trump faces from misdemeanors to felonies punishable by up to four years in prison.

Trump has pleaded not guilty to all 34 counts and denies having had a sexual encounter with Daniels. He argues the case is a politically motivated attempt to interfere with his campaign. Bragg, the prosecutor, is a Democrat.

"Fat Alvin, corrupt guy," Trump said of Bragg at a New Jersey political rally on Saturday night.

Prosecutors say the payment to Daniels was part of an illegal scheme to influence the 2016 election by buying the silence of people with potentially damaging information. Trump's lawyers say it was to spare him embarrassment with his family. 

Cohen in 2018 pleaded guilty to violating federal campaign finance law with the payment to Daniels. He testified that Trump directed him to make the payment. Prosecutors in that federal case never charged Trump with a crime.  

Trump's defense lawyers have told the 12 jurors and six alternates that Cohen is a liar and cannot be believed. They say he acted on his own when paying Daniels and seek to distance Trump from the reimbursement checks and invoices at the heart of the case.

Cohen has admitted to lying under oath multiple times, providing substantial fodder for the defense to undermine his credibility.

He admitted to lying to the U.S. Congress in 2017 about a Trump Organization real estate project in Moscow, but has since said he did so to protect Trump.

He also pleaded guilty to violating tax law in 2018, but now says he did not commit that crime.

COHEN A TARGET OF TRUMP ANGER

During testimony last week jurors saw the 34 invoices, corporate ledger entries and checks that prosecutors say were falsified by Trump to obscure his reimbursement to Cohen.

A former Trump employee testified he had been told by Trump's top financial officer that the reimbursements to Cohen were for expenses incurred during the campaign. That could undercut an argument made by Trump's lawyers that the payments were for legal work.

However, neither that employee nor another who testified last week was able to say whether Trump himself directed the paper trail to be falsified to hide the payment to Daniels - a hole that prosecutors will aim to fill with Cohen's testimony.

Cohen has been on the receiving end of Trump's vitriolic social media attacks, some of which the judge has said violated a gag order restricting Trump from making statements about jurors, witnesses and families of the judge and prosecutors if meant to interfere with the case.

Trump has called the gag order a violation of his right to free speech, and has said it is unfair to bar him from responding to attacks by witnesses such as Cohen and Daniels. 

Merchan has fined Trump $10,000 for 10 violations of the order and warned the former president he could face time in jail if he keeps up his attacks.   

Prosecutors planned to call two witnesses on Monday and predicted they could rest their case by the end of this week.

The case is widely seen as less consequential than three other criminal prosecutions Trump faces, but it is the only one certain to go to trial before the election.

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 Luc Cohen in New York; Editing by Noeleen Walder and Howard Goller)

By Luc Cohen