The judge said that “therefore, going forward, this court will have to consider jail.”

That warning preceded a desert-dry description of a $130,000 payment made by Mr. Trump’s fixer, Michael D. Cohen, to a porn star to buy her silence about her story of a one-night stand in 2006. That testimony was elicited from two employees, one current and one retired, of the Trump Organization.

The former president is accused of falsifying business records to cover up that $130,000 payment. Mr. Trump, 77, the first American president to face criminal prosecution, has denied the charges and says he did not have sex with the woman, Stormy Daniels. If convicted, he could face prison time or probation.

Here are five takeaways from Mr. Trump’s 12th day on trial:

Trump’s comments continue to bring him trouble.

Justice Merchan once again found Mr. Trump in contempt for violating his gag order that prohibited attacks on jurors, witnesses, court staff and others. It was Mr. Trump’s 10th violation, and resulted from an April 22 interview in which he said the jury was from “a purely Democrat area,” calling those circumstances “very unfair.”

The violation resulted in a $1,000 fine, which was on top of the $9,000 he was fined last week for nine other violations, a pittance for a billionaire. But the comments by Justice Merchan, who called the repeated violations “a direct attack on the rule of law,” may have a far greater impact.

Justice Merchan said he recognized what a disruption and dramatic decision jailing Mr. Trump would be, but made clear he would imprison him, if necessary.

“You are the former president,” the judge said, adding, “and possibly the next president.”

But, he added, “at the end of the day, I have a job to do.”

A chastised Mr. Trump seethes.

As Justice Merchan admonished Mr. Trump, the former president hunched over the defense table staring at him. When he finished, Mr. Trump shook his head. After court, Mr. Trump again blasted the trial as “election interference” in comments in the hallway outside the courtroom.

“They want to keep me off the trail,” said Mr. Trump, the presumptive Republican nominee, adding he had hoped the trial might end on Monday.

Mr. Trump did perk up when Jeffrey S. McConney, the Trump Organization’s former corporate controller, recounted when Mr. Trump told him — as a joke — that he was fired because of a dip in his cash balances. Mr. Trump smiled at the memory.

The jury saw evidence of Michael D. Cohen’s payments.

Mr. McConney testified about reimbursing Mr. Cohen for his payment to Ms. Daniels. (He was ultimately paid $420,000, which included a bonus and money to offset taxes, an arrangement outlined in a handwritten note by Mr. McConney shown to jurors.)

Paid over a year, the hush money amounted to $35,000 a month. The schedule was painstakingly documented by prosecutors, who showed invoices Mr. Cohen sent to the Trump Organization and checks to Mr. Cohen. Mr. Cohen’s invoices mention a “retainer agreement,” something that prosecutors say didn’t exist, but rather was a way to disguise the reimbursement.

On Monday, Mr. McConney was asked if he saw such an agreement.

“I did not,” Mr. McConney replied.

It was dull, but it was important.

Mr. McConney and another witness — Deborah Tarasoff, an accounts supervisor at the Trump Organization — both testified about internal machinations to pay back Mr. Cohen.

Prosecutors asked them to identify a raft of business records and other forms to authenticate and describe them. After a week that had set a sensational backdrop for the case — including the presidential race in 2016, and the frantic efforts to keep stories about extramarital affairs quiet — Monday’s testimony was often soporific.

Notably, Ms. Tarasoff also testified about the checks to Michael Cohen, which account for business records, which prosecutors say were falsified.

But the slog was necessary for prosecutors hoping to hang a case on documents. Still, Mr. Trump didn’t seem impressed, keeping his eyes closed for some of it, as he has for swaths of testimony on other days.

The week — and weeks — ahead may get more exciting.

Despite Mr. Trump’s shut eyes, his trial has featured gripping testimony about tabloid deals, celebrity shakedowns and life inside his inner circle. The rest of the week may intensify, including a possible appearance by Ms. Daniels.

She will likely attract a crush of media attention, driven by the spectacle of her sitting just across from Mr. Trump. With eight days of testimony complete, prosecutors said on Monday that they would need approximately two weeks to finish their case. The defense then gets its turn, and closing arguments follow.

After that, the jury will take over.