The star prosecution witness in Donald Trump’s hush money trial, Michael Cohen, took the stand on Monday, poised to offer testimony that could help shape the outcome of the first criminal case against a US president.

Mr Cohen, Trump’s former lawyer and personal fixer, is by far the Manhattan district attorney’s most important witness in the case, and his appearance signals that the trial is entering its final stretch.

Prosecutors say they may wrap up their presentation of evidence by the end of the week.

The testimony of a witness with such intimate knowledge of Trump’s activities could heighten the legal exposure of the presumptive Republican presidential nominee if jurors deem him sufficiently credible.

But politically, prosecutors’ reliance on a witness with such a chequered past — Mr Cohen pleaded guilty to federal charges related to the payments and to lying to Congress — could be a boon for Trump as he raises money off his legal woes and paints the case as the product of a tainted criminal justice system.

Mr Cohen is expected to testify about his role in arranging hush money payments on Trump’s behalf during his first presidential campaign, including to porn actor Stormy Daniels, who told jurors last week that the 130,000 dollars that she received in 2016 was meant to prevent her from going public about a sexual encounter she says she had with Trump in a hotel suite a decade earlier.

He also matters because the reimbursements he received form the basis of the charges against Trump — 34 felony counts of falsifying business records.

Prosecutors say the reimbursements were logged as legal expenses to conceal the payments’ true purpose in what they allege was an effort to illegally interfere in the 2016 presidential election.

Trump did not appear to make eye contact with Mr Cohen as the lawyer began his testimony by recounting his upbringing on Long Island as the son of a Holocaust survivor and his career experience with Trump.

Defence lawyers have teed up a bruising cross-examination of Mr Cohen, telling jurors during opening statements that the fixer-turned-foe is an “admitted liar” with an “obsession to get President Trump”.