During E. Jean Carroll’s first day on the witness stand, her lawyer asked what had brought her to a federal courtroom in Manhattan.

“I am here because Donald Trump raped me and when I wrote about it, he said it didn’t happen,” Ms. Carroll replied. “He lied and shattered my reputation, and I am here to try to get my life back.”

A day later, Mr. Trump, who has denied the attack and called Ms. Carroll a liar, campaigned in New Hampshire, joking to a crowd about his changing nicknames for Hillary Clinton and President Biden. He did not mention Ms. Carroll’s testimony, or the civil trial going on 250 miles away. But he remarked cheerfully on a poll released that day, which showed him far and away leading the 2024 Republican primary field.

Since Mr. Trump was indicted last month in a criminal case brought by the Manhattan district attorney’s office, his legal travails and his third presidential campaign have played out on a split screen. The courtroom dramas have taken place without news cameras present, even as the race has returned Mr. Trump to the spotlight that briefly dimmed after he left the Oval Office.

Ms. Carroll’s harrowing testimony, a visceral demonstration of Mr. Trump’s legal peril, has emphasized the surreal nature of the divide. Mr. Trump is the front-runner for the 2024 Republican presidential nomination. But he has also been indicted on 34 felony false records charges, and in Ms. Carroll’s case faces a nine-person jury that will determine whether he committed rape decades ago. And then there are the other investigations: for election interference, mishandling sensitive documents and his role in the attack on the U.S. Capitol.

“To see a former and potential future president of the United States confront all these legal issues at once is bizarre,” said Jennifer Horn, a former chairwoman of the New Hampshire Republican Party and a vocal opponent of Mr. Trump. “But what’s really disturbing about it is that he’s the front-runner for a major political party in this country. And you can’t just blame that on him. You have to blame that on the leaders of the party and their primary base.”

The past week brought the former president a steady stream of setbacks. Ms. Carroll gave detailed and graphic testimony about the encounter with Mr. Trump. The judge in the case sought to limit Mr. Trump’s posts on social media, as did the Manhattan district attorney’s office in its own case. And former Vice President Mike Pence testified before a grand jury hearing evidence about Mr. Trump’s attempts to overturn the results of the 2020 election.

Mike Murphy, a Republican political strategist who advised John McCain and Jeb Bush, said that trials and investigations of Mr. Trump often create “a psychological roller coaster for Trump-hating Democrats,” giving them hope that he will be taken down, only to leave them disappointed. Mr. Trump’s legal problems have yet to create significant political problems given the unflinching loyalty of his core supporters.

Since Mr. Trump was indicted, his poll numbers have risen. Criminal investigations against him, in Georgia and Washington, as well as Ms. Carroll’s trial and a civil fraud lawsuit brought by the New York Attorney General’s office, have done little to hamper him with his supporters. The poll he mentioned Thursday predicted that he would receive 62 percent of the vote in the Republican primary. His closest opponent, Gov. Ron DeSantis of Florida, who has not yet declared that he is running, was polling at 16 percent.

But the investigations could cause Mr. Trump real harm. If he is convicted in Manhattan, where he pleaded not guilty to 34 felony charges of falsifying business records, he could face up to four years in prison. Criminal charges in Georgia and Washington could come with steeper punishments. And the New York attorney general’s lawsuit against him — which accused him of deceiving lenders and insurers by fraudulently overvaluing his assets — could exact a heavy financial toll.

No matter the outcome, any direct connection between Mr. Trump’s legal fate in the rape case and his political fortunes is tenuous. But Ms. Carroll’s lawyers have reshaped a political bombshell from 2016 into a potent legal weapon: They plan to use the “Access Hollywood” tape on which Mr. Trump boasts about grabbing women by the genitals as the basis for a compelling story about a self-styled playboy man-about-town whose modus operandi was assaulting women.

Mr. Trump said on the tape that “when you’re a star, they let you do it.” When the comments became public during the 2016 election, Mr. Trump characterized them as “locker room banter” and after his victory, they became an example of his apparent immunity to scandal.

In the courtroom, which Mr. Trump has avoided, Ms. Carroll’s team argued that his words were not to be dismissed, even years after they became public.

“This was not locker room talk,” Shawn G. Crowley, one of her lawyers, said on the first day of the trial. “It’s exactly what he did to Ms. Carroll and to other women.” Those other women, named by Mr. Crowley as Jessica Leeds and Natasha Stoynoff, are expected to testify for Ms. Carroll.

Ms. Carroll described for the jury how a chance meeting with Mr. Trump in a Bergdorf Goodman store in New York City in the mid-1990s turned into an encounter that would haunt her for nearly 30 years.

The assault, she said, took place in a sixth-floor dressing room. After Mr. Trump shut the door, she said, he shoved her against the wall hard enough that she hit her head. He put his mouth against hers, Ms. Carroll said, pulled down her tights, put his fingers inside of her vagina and then his penis.

Ms. Carroll testified that she has been unable to have sex or a romantic relationship since.

“I am a happy person basically, but I’m aware that I have lost out on one of the glorious experiences of any human being,” she said. “Being in love with somebody else, making dinner with them, walking the dog together. I don’t have that.”

On Thursday, a lawyer for Mr. Trump, Joseph Tacopina, began cross-examining Ms. Carroll. He sought to highlight her memory lapses — most prominently her inability to provide the date of the alleged rape — and to elicit testimony that he suggested showed the jury how implausible the story seemed. He noted that Ms. Carroll herself had written in her 2019 memoir, where she first told her story, that some of the details were “odd” and “inconceivable.”

“It all comes down to: Do you believe the unbelievable?” Mr. Tacopina said during his opening statement.

The jurors, six men and three women, will decide whether Ms. Carroll proved by a preponderance of the evidence that Mr. Trump committed the assault. Because the trial is civil, he would face monetary damages rather than a conviction.

Ms. Carroll’s lawsuit also seeks a retraction of what the suit says were defamatory comments on his Truth Social website last October, when he called her case “a complete con job” and “a Hoax and a lie.”

The social platform is one of Mr. Trump’s most potent political tools. And the legal cases are threatening his ability to use it unhindered.

On Tuesday, a prosecutor working for the Manhattan district attorney, Alvin L. Bragg, asked the state judge in the false business records criminal case to limit Mr. Trump from disseminating evidence online, including on that platform.

And on Wednesday, after the former president made Truth Social posts calling Ms. Carroll’s allegation a “made up SCAM” and a “fraudulent & false story,” the federal judge, Lewis A. Kaplan, said they were “entirely inappropriate” and suggested the former president was attempting to influence the jury.

When, hours later, Judge Kaplan was told by Ms. Carroll’s lawyer that Mr. Trump’s son Eric had tweeted an attack on Ms. Carroll’s case, the judge sternly suggested the former president might face criminal sanction.

“If I were in your shoes,” Judge Kaplan told Mr. Tacopina, “I’d be having a conversation with the client.”

Kate Christobek contributed reporting.