NEW YORK — The sordid details of shady backroom dealings spilled out in open court Tuesday when ex-National Enquirer publisher David Pecker confessed his magazine used “checkbook journalism” to pay off sources with unsavory stories about Donald Trump.

The longtime executive admitted to a judge and a jury that his job was to “catch and kill” bad press centered on Trump, including allegations of affairs and children born out of wedlock.

As the Manhattan district attorney’s lead witness, Pecker’s testimony bolstered the theory at the heart of the state’s case — that Trump and his allies attempted to influence the outcome of the 2016 election by burying bad news about Trump while elevating wild claims that were embarrassing to his political opponents.

That alleged conspiracy underpins the 34 felony counts of falsifying business records Trump faces in connection with one of the hush money deals. He’s pleaded not guilty to the charges.

And there’s more to come. Pecker is again expected to take the stand Thursday when the trial will next be held.

Pecker’s testimony Tuesday started with the story of his relationship with Trump, dating back to the late 1980s when they were first introduced at Mar-a-Lago, which he pinpointed as when their “great, mutually beneficial relationship” began.

Early on, before Pecker acquired the National Enquirer, he pitched the then-New York business mogul on a magazine called “Trump Style,” an idea Trump apparently liked that was made a reality. From there, Trump would tip Pecker off about news from his show, “The Apprentice,” which Pecker’s magazine readers would “religiously” follow.

Their relationship evolved after Trump announced his first presidential bid in 2015. Pecker began to see Trump “more frequently,” and they began to speak by phone every few weeks.

The tabloid publisher’s communication with Trump’s then-personal attorney and fixer, Michael Cohen, also sharply increased, growing from “once a month or twice a quarter” to “at minimum every week.”

“And if there was an issue, it could be daily,” Pecker said.

A pivotal moment came in August 2015, two months after Trump announced his candidacy, when Cohen called Pecker to tell him that “the boss” — a term he used for Trump — wanted to meet. Pecker met with Trump and Cohen at Trump Tower, where they asked him how he could “help the campaign,” he said.

Pecker offered to publish positive stories about Trump and negative stories about the candidate’s 2016 presidential opponents. He also agreed to serve as the campaign’s “eyes and ears.” 

The deal was of “mutual benefit,” he said.

The agreement wasn’t formalized in writing — “It was just an agreement among friends,” Pecker said — but it soon took effect. He told his subordinates at American Media Inc., where he was CEO and which published the National Enquirer, to keep the deal a secret. 

“[I was] going to try to help the campaign, and to do that, I wanna keep this as quiet as possible,” Pecker said.

In the lead up to the campaign, the National Enquirer ran a slew of unseemly and baseless stories about Trump’s opponents. Their headlines spanned a purported picture of Sen. Ted Cruz’s (R-Texas) father with President Kennedy’s assassin, to a supposed “love child” of Sen. Marco Rubio (R-Fla.).

Many of the stories were directly planted by Cohen, who would send shady stories about Trump’s political adversaries depending on the strength of their debate performances. His tips would form the basis of the Enquirer’s articles, Pecker said.

The tabloid publisher also kept anti-Trump stories from coming to light.  

Pecker testified that because Trump was running for public office and “well-known as the most eligible bachelor,” he expected that women from Trump’s past might try to sell their stories to magazines.

But the first story he ever killed for Trump actually came from a Trump employee.

After a tipster informed the Enquirer that Trump Tower doorman Dino Sajudin was claiming Trump fathered an illegitimate child with another building employee, Pecker said he “immediately” called Cohen, who said he would look into it.

Even after determining  Sajudin’s story was false, the Enquirer still chose to purchase rights to his story for $30,000 to ensure other media outlets weren’t tipped off — a rate far higher than offered for the average story, Pecker said.

“So this was a way to lock it up?” prosecutor Joshua Steinglass asked. 

“Yes,” Pecker replied, adding that even though the story was false, it would have been “very embarrassing to the campaign” if it got out. 

When Pecker agreed to buy the story, Cohen told him “the boss” would be “very pleased.” 

Pecker at one point tried to release the doorman from his exclusivity deal, but said Cohen promptly shut it down.

“He said, ‘When?’ I said, ‘Now,’” Pecker said. “He said, ‘No, release him after the election.’” 

When Pecker testifies again, he’ll likely detail two other catch-and-kills involving ex-Playboy model Karen McDougal and porn actress Stormy Daniels. Both claimed they had affairs with Trump, which he has denied, and both were paid off to stay quiet about the allegations.

Once Pecker’s direct examination concludes, Trump attorneys will attempt to poke holes in his testimony and undermine any evidence that fortifies the prosecution’s case on cross-examination.