Former U.S. President Donald Trump was intimately involved with all aspects of a scheme to stifle stories about sex that threatened to torpedo his 2016 campaign, his former lawyer said Monday in matter-of-fact testimony that went to the heart of the former president’s hush money trial.

“Everything required Mr. Trump’s sign-off,” said Michael Cohen, Mr. Trump’s fixer-turned-foe and the prosecution’s star witness in a case now entering its final, pivotal stretch.

In hours of highly anticipated testimony, Mr. Cohen placed Mr. Trump at the centre of the hush money plot, saying the then-candidate had promised to reimburse the lawyer for the money he fronted and was constantly updated about behind-the-scenes efforts to bury stories feared to be harmful to the campaign.

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“We need to stop this from getting out,” Mr. Cohen quoted Mr. Trump as telling him in reference to porn actor Stormy Daniels’ account of a sexual encounter with Mr. Trump a decade earlier. The then-candidate was especially anxious about how the story would affect his standing with female voters.

A similar episode occurred when Mr. Cohen alerted Mr. Trump that a Playboy model was alleging that she and Mr. Trump had an extramarital affair. “Make sure it doesn’t get released,” was Mr. Cohen’s message to Mr. Trump, the lawyer said. The woman, Karen McDougal, was paid $150,000 in an arrangement that was made after Mr. Trump received a “complete and total update on everything that transpired.”

“What I was doing, I was doing at the direction of and benefit of Mr. Trump,” Mr. Cohen testified.

Mr. Trump has pleaded not guilty and denied having sexual encounters with the two women.

Mr. Cohen is by far the prosecution’s most important witness, and though his testimony lacked the electricity that defined Ms. Daniels’ turn on the stand last week, he nonetheless linked Mr. Trump directly to the payments and helped illuminate some of the drier evidence such as text messages and phone logs that jurors had previously seen.

The testimony of a witness with such intimate knowledge of Mr. Trump’s activities could heighten the legal exposure of the presumptive Republican presidential nominee if jurors deem him sufficiently credible. But prosecutors’ reliance on a witness with such a checkered past — Mr. Cohen pleaded guilty to federal charges related to the payments — also carries sizable risks with a jury and could be a boon to Mr. Trump politically as he fundraises off his legal woes and paints the case as the product of a tainted criminal justice system.

The men, once so close that Mr. Cohen boasted that he would “take a bullet” for Mr. Trump, had no visible interaction inside the courtroom. The sedate atmosphere was a marked contrast from their last courtroom faceoff, when Mr. Trump walked out of the courtroom in October after his lawyer finished questioning Mr. Cohen during his civil fraud trial.

This time around, Mr. Trump sat at the defence table with his eyes closed for long stretches of testimony as Mr. Cohen recounted his decade-long career as a senior Trump Organization executive, doing work that by his own admission sometimes involved lying and bullying others on his boss’s behalf.

Jurors had previously heard from others about the tabloid industry practice of “catch-and-kill,” in which rights to a story are purchased so that it can then be quashed. But Mr. Cohen’s testimony, which continues Tuesday, is crucial to prosecutors because of his direct communication with the then-candidate about embarrassing stories he was scrambling to suppress.

Mr. Cohen also matters because the reimbursements he received from a $130,000 hush money payment to Ms. Daniels, which prosecutors say was meant to buy her silence in advance of the election, form the basis of 34 felony counts charging Trump with falsifying business records. Prosecutors say the reimbursements were logged, falsely, as legal expenses to conceal the payments’ true purpose. Defence lawyers say the payments to Mr. Cohen were properly categorised as legal expenses.

Under questioning from a prosecutor, Mr. Cohen detailed the steps he took to mask the payments. When he opened a bank account to pay Ms. Daniels, an action he said he told Mr. Trump he was taking, he told the bank it was for a new limited liability corporation but withheld the actual purpose.

“I’m not sure they would’ve opened it,” he said, if they knew it was ”to pay off an adult film star for a nondisclosure agreement.”

To establish Mr. Trump’s familiarity with the payments,Mr. Cohen told the jury that Mr. Trump had promised to reimburse him. The two men even discussed with Allen Weisselberg, a former Trump Organization chief financial officer, how the reimbursements would be paid as legal services over monthly instalments, Mr. Cohen testified.

And though Mr. Trump’s lawyers have said he acted to protect his family from salacious stories, Mr. Cohen described Mr. Trump as preoccupied instead by the impact they would have on the campaign.

He said Mr. Trump even sought to delay finalising the Ms. Daniels transaction until after Election Day so he wouldn’t have to pay her.

“Because,” Mr. Cohen testified, “after the election it wouldn’t matter” to Mr. Trump.

Mr. Cohen also gave jurors an insider account of his negotiations with David Pecker, the then-publisher of the National Enquirer, who was such a close Mr. Trump ally that Mr. Pecker told Mr. Cohen his publication maintained a “file drawer or a locked drawer” where files related to Mr. Trump were kept.

That effort took on added urgency following the October 2016 disclosure of an “Access Hollywood” recording in which Mr. Trump was heard boasting about grabbing women sexually.

The Daniels payment was finalised several weeks after that revelation, but Monday’s testimony also centred on a deal earlier that fall with Ms. McDougal.

Mr. Cohen testified that he went to Mr. Trump immediately after the National Enquirer alerted him to a story about the alleged Ms. McDougal affair. “Make sure it doesn’t get released,” he said Mr. Trump told him.

Mr. Trump checked in with Mr. Pecker about the matter, asking him how “things were going” with it, Mr. Cohen said. Mr. Pecker responded, ‘We have this under control, and we’ll take care of this,” Mr. Cohen testified.

Mr. Cohen also said he was with Mr. Trump as he spoke to Mr. Pecker on a speakerphone in his Trump Tower office.

“David had stated that it’s going to cost them $150,000 to control the story,” Mr. Cohen said. He quoted Mr. Trump as saying: “No problem, I will take care of it,” which Mr. Cohen interpreted to mean that the payment would be reimbursed.

To lay the foundation that the deals were done with Mr. Trump’s endorsement, prosecutors elicited testimony from Mr. Cohen designed to show Trump as a hands-on manager. Acting on Trump’s behalf, Cohen said, he sometimes lied and bullied others, including reporters.

“When he would task you with something, he would then say, ‘Keep me informed. Let me know what’s going on,’” Mr. Cohen testified. He said that was especially true “if there was a matter that was troubling to him.”

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

Prosecutors aim to blunt those attacks by acknowledging Mr. Cohen’s past crimes to jurors and by relying on other witnesses whose accounts, they hope, will buttress Mr. Cohen’s testimony. They include a lawyer who negotiated the hush money payments on behalf of Ms. Daniels and Ms. McDougal, as well as Mr. Pecker and Ms. Daniels.

After Mr. Cohen’s home and office were raided by the FBI in 2018, Mr. Trump showered him with affection on social media and predicted that Mr. Cohen would not “flip.” Months later, Mr. Cohen did exactly that, pleading guilty to federal campaign-finance charges.

Besides pleading guilty to the hush money payments, Mr. Cohen later admitted lying to Congress about a Moscow real estate project that he had pursued on Mr. Trump’s behalf during the heat of the 2016 campaign. He was sentenced to three years in prison, but spent much of it in home confinement.