Hallie Biden walked briskly to the witness box, past her brother-in-law and ex-boyfriend Hunter Biden, to chronicle a short, star-crossed relationship that ended in anguish, her own addiction and, eventually, his criminal prosecution.

Ms. Biden, 50, is by far the most important witness for the prosecution. She is one of the few people with the knowledge to offer a detailed, and intimate, accounting of Mr. Biden’s all-consuming addiction to crack cocaine in the fall of 2018. He is on trial on charges of lying about his drug use on a form when buying a gun in October 2018 and of illegally possessing the weapon.

Moments after sitting down, she delivered what seemed to be the heaviest blow yet to Mr. Biden’s defense, confirming that he bought and smoked crack in the 48 hours after he purchased a gun in Delaware.

But if the purpose of her appearance was to nail down a dry, prosecutorial timeline, the effect on Ms. Biden was of forcing a recovering addict to revisit days of desperation and shame. She was clearly shaken by the task, repeatedly scanning the visitors’ gallery for friendly faces during breaks in her testimony.

“It was a terrible experience that I went through,” she said.

The defendant nodded in affirmation, almost imperceptibly, as she spoke.

Ms. Biden — speaking in nervous, clipped bursts — admitted to the jury that she had smoked crack after President Biden’s youngest son had introduced her to the drug in the summer of 2018. She said she was “ashamed” and embarrassed by their behavior when they briefly shared a house in Annapolis, Md., when both were still reeling from the death of her husband and his brother, Beau Biden, from brain cancer in 2015.

The texts between the two were wrenching. The lead prosecutor in the case, Leo J. Wise, who normally speaks with an unhurried, high-volume cadence, seemed to lower his voice and rush his delivery to read their raw, frantic conversations.

The exchanges alternated between recrimination and affection, with Ms. Biden begging him to seek treatment, and not to cheat on her, as he trawled the streets for drugs.

And there were a lot of drugs. Mr. Biden, she said, bought multiple rocks of crack in Washington, where he kept an apartment — some the size of “Ping-Pong balls, or bigger maybe” — and stored them in his “backpack or car.”

Two transactions seemed to be particularly damaging to Mr. Biden’s defense, which is predicated on the assertion that he was not taking drugs around the time he signed the federal screening form to buy a Colt handgun in Wilmington on Oct. 12, 2018.

The day after, he texted Ms. Biden that he was “buying.” That indicated he was purchasing crack, she told the court.

In a second message, from late on the night of Oct. 14, Mr. Biden acknowledged he was “sleeping on a car” and “smoking crack” behind the minor league baseball stadium in Wilmington, after buying drugs from a dealer named Mookie.

It was part of a pattern of erratic behavior, she added, saying that he would be unreachable for weeks at a time and that she or her children would scrounge through his car for drugs or alcohol to help him “start anew and deal with stuff” when he reappeared exhausted at her home.

On Oct. 23, 2018 — 11 days after Mr. Biden bought the gun — Ms. Biden confiscated the weapon, drove it to a high-end supermarket in Delaware and tossed it in a trash can, hoping he would never discover who had taken it.

But it was quickly retrieved by the police, who contacted them, spurring a series of panicked communications from Mr. Biden. He seemed to have immediately grasped the dire implications. He cursed her out and called her stupid, according to the texts.

“I’ll take the blame,” replied Ms. Biden, who had repeatedly urged him to go to rehab and seemed to view her actions as an intervention of sorts. “I don’t want to live like this.”

One of the most damaging aspects of her testimony, which came on the fourth morning of Mr. Biden’s quickly moving trial, was her claim that he took few precautions in storing the gun when it was in his possession. Abbe Lowell, Mr. Biden’s lawyer, claimed in his opening arguments that Mr. Biden had kept the gun in a “lock box” in his truck and took it out only once in the time he owned it.

But the government produced a text from Ms. Biden chiding her boyfriend at the time, telling him the box had been left open in an unlocked vehicle, “windows down.” She warned Mr. Biden that “the kids search your car.”

When she searched the car on Oct. 23, she noticed “a dusting of powder” that she assumed to be “remnants of crack cocaine” before finding the gun in a case with a broken lock. Prosecutors then showed surveillance video of her tossing the gun only to return later and frantically try to recover it.

“I realize it was a stupid idea now, but I was just so panicked,” she said.

Earlier, Mr. Biden’s defense team suggested that it would try to challenge Ms. Biden’s credibility on cross-examination by introducing new text messages — some “salacious” — showing that she was angry at him over his infidelity and drug abuse. His lawyers have also signaled that he might have lied to Ms. Biden in texts, saying he was buying drugs to cover up his affairs.

At times, Mr. Biden’s web of romantic intrigue and obliteration of personal boundaries bordered on the comical. At one point, he even used the cellphone number of his estranged wife, Kathleen Buhle, to text with Ms. Biden.

“That freaks me out,” she wrote.

“This is Kathleen and I’m gonna beat you up,” he joked.

David C. Weiss, the special counsel who has brought a separate case against Mr. Biden involving more serious tax offenses, has turned to the women closest to Mr. Biden to document his drug use, revisiting some of the most damaging episodes in the Biden family’s recent history as the campaign season intensifies.

On Wednesday, two of Mr. Biden’s former romantic partners, his former wife and a former girlfriend, provided vivid testimony about his addiction to crack in the weeks and months before he applied for a gun.

Almost all the events at issue in the trial happened in 2018, when Joseph R. Biden Jr. was out of office.

Mr. Lowell has suggested he could seek to undercut Ms. Biden’s narrative.

Mr. Biden is charged with three felonies: lying to a federally licensed gun dealer, making a false claim on the federal firearms application and possessing an illegally obtained gun in October 2018. If convicted, Mr. Biden could face up to 25 years in prison and $750,000 in fines. But nonviolent first-time offenders who have not been accused of using the weapon in another crime rarely receive serious prison time for the charges.

The government’s case turns on a relatively straightforward question: whether Mr. Biden was abusing drugs when he filled out the federal firearms application claiming he was not an “unlawful user” of controlled substances. “Addiction may not be a choice, but lying and buying a gun is a choice,” Derek Hines, a top deputy to Mr. Weiss, told jurors in opening statements on Tuesday.

The sheer amount of unflattering evidence assembled by Mr. Weiss is intended to prove that Mr. Biden knowingly lied when he claimed not to be taking drugs when he bought the handgun.

But it has, in the view of even some Biden family critics, moved far beyond that goal — into a publicly humiliating trial of the president’s troubled son for an offense that, while a crime, is seldom prosecuted as a stand-alone charge for someone with no prior criminal record who has been sober for years.