The trial of Sean Combs ended this summer with the music mogul brought to his knees by emotion after he avoided the possibility of spending the rest of his life in prison.

But he was not a free man.

Though the jury had cleared him of charges that he forcibly sex trafficked his girlfriends and ran a racketeering conspiracy, it found him guilty of two lesser counts related to elaborate voyeuristic sex marathons at the heart of the case.

Those convictions are based on a federal law known as the Mann Act, which since 1910 has made it a federal offense to transport people across state lines for the purposes of prostitution.

Once treated as something of a side issue in a case built on some of the most serious criminal statutes on the books, the Mann Act charges are now the main battleground for the continuing legal fight over Mr. Combs’s fate as his Oct. 3 sentencing nears.

For prosecutors, the law represents the last opportunity to salvage significant prison time for a man that they portrayed not as some regular customer of prostitutes, but the violent abuser of women, who orchestrated the sex acts.

For Mr. Combs’s lawyers, the convictions — based on a law originally enacted as the White Slave Traffic Act — are all that is left of what they have characterized as an overreaching prosecution that sought to criminalize kinky but consensual sexual activity.

“Once you accept the verdict,” said Alexandra Shapiro, a lawyer for Mr. Combs, in a recent interview, “what you’re left with is consensual sexual activity by adults who were fully capable of deciding whether or not to engage in the activity.”

A century removed from the urban brothels that helped inspire the law, Mr. Combs was convicted of hiring men — often through an upscale online escort service — to fly around the country to perform sexually in luxury hotel rooms with, at different times, two of the mogul’s girlfriends.

The women testified that they felt manipulated, unable to say no to the repeated encounters, known as “freak-offs” and “hotel nights,” whether cowed by a pattern of physical abuse or Mr. Combs’s financial leverage.

Because the jury did not find that the women had been forced or coerced, Mr. Combs is facing possibly years — rather than life — in prison for what his lawyers have framed as a swingers lifestyle.

Prosecutors have depicted his conduct in significantly darker terms, focusing on the frequency of the freak-offs, the copious drug use involved, and the accounts of violence toward the women to assert Mr. Combs was anything but the low-level “john” conjured by the defense.

“Far from being an ordinary and casual consumer of commercial sex,” prosecutors wrote in a court filing after the verdict, “the defendant transported individuals for the purpose of prostitution on hundreds of occasions over the course of decades.”

The law responsible for Mr. Combs’s conviction has had a winding and at times fraught history. Born in an era marked by moral panic, it has been the legal underpinning for other high-profile sex scandals and was amended in law and in practice into a modern law enforcement tool, one that played a role in several major #MeToo-era prosecutions.

The law emerged from racist and nativist fears over “white slavery” and Progressive-era anxieties that young women in cities were being lured into prostitution, said Jessica Pliley, the author of the book “Policing Sexuality: The Mann Act and the Making of the FBI.” The law’s main sponsor was Representative James R. Mann of Illinois, where a well-known Chicago red-light district had become a major concern of social reformers and public officials.

It drew its power in part from its vagueness. The text made it illegal not only to transport a woman or girl for the purposes of prostitution, but also for “debauchery” or “any other immoral purpose.” The law led to expansive F.B.I. crackdowns on interstate prostitution, as well as investigations into many other kinds of sexual conduct.

Federal agents focused on sexual abuse of women and girls, but also pursued investigations into reports of adultery, bigamy and interracial relationships, driven by citizens seeking to call out what they deemed to be immoral behavior.

“When you look at the enforcement of the Mann Act, it provides a lens for revealing what the sexual, gender, racialized preoccupations are of the given moment,” Professor Pliley said in an interview.

Perhaps the most infamous Mann Act case was the prosecution of Jack Johnson, the first Black heavyweight boxing champion, who was convicted in 1913 of transporting a white woman — his lover, who worked as a prostitute — across state lines. Widely viewed as having been the victim of a racially motivated prosecution, Johnson was pardoned posthumously by President Trump in 2018.

The law became the foundation for other headline-grabbing cases.

Charlie Chaplin, who had been dogged by the F.B.I. under suspicion of communist sympathies, was brought to trial in 1944 on charges that he had transported an aspiring actress to New York by train for “immoral purposes.” He was acquitted after a case in which Chaplin testified that all he and the actress had done in the Waldorf Astoria — the hotel said to have been a central scene of the crime — was engage in small talk, according to a book chronicling the case.

And in the 1960s, Chuck Berry spent 20 months in prison after he was convicted of transporting a 14-year-old girl, whom he had met while on tour, with the intent to induce her to engage in “immoral practices.”

That expansive immorality clause was excised in a 1986 amendment by Congress, but its replacement — “any sexual activity for which any person can be charged with a criminal offense” — still allowed for prosecution of a broad set of sexual offenses.

Over time, the government’s use of the law evolved to focus on the types of cases more familiar to prosecutors today: trafficking and sex crimes against underage victims.

The passage of the Trafficking Victims Protection Act in 2000 — now considered the country’s main sex trafficking law — altered the use of the Mann Act even further.

“To charge someone with the Mann Act today is not at all the same as having done so in the early 20th century,” said Mary Graw Leary, a former federal prosecutor and human trafficking scholar. “We live in a different world.”

Now, legal experts say, the government often deploys Mann Act charges in tandem with the more severe sex trafficking law because they are significantly easier to prove and serve as something of a backstop in challenging cases.

The Combs case shows the usefulness of that strategy for prosecutors, one of whom told the jury during the trial that “even if all the participants enthusiastically consented,” they could still convict under the Mann Act.

Mann Act charges are often used as building blocks for prosecutors to assemble a complex case with a sweeping narrative of behavior, experts say, as in the federal cases against R. Kelly and Ghislaine Maxwell.

“It is not typically the star of the show, except for in unique circumstances,” said Margaret Gandy, a former federal prosecutor, “but it is an important tool toward building the narrative of the operation of sex trafficking or racketeering.”

Before Mr. Combs went to trial, his lawyers had seized on the lower points of the law’s history, citing the Johnson case, for example, as part of an argument that the Mann Act was continuing to be used to target “Black male sexuality.” They noted in court papers the lack of charges against Eliot Spitzer, who resigned as governor of New York after he was found to have patronized a high-priced prostitution ring and was investigated for possible Mann Act violations.

Judge Arun Subramanian, who has overseen the Combs case, rejected the argument that Mr. Combs was being unfairly prosecuted based on his race, writing that “whatever the troubling history” of the Mann Act, “its present-day enforcement appears on its face race-neutral in this district, reaching across race and gender.”

After the eight-week trial finished in July, Mr. Combs’s lawyers made a series of filings in which they argued that the circumstances of the mogul’s case were a conspicuous outlier in the modern-day scope of Mann Act prosecutions and should never have been charged under the law at all.

The defense began an effort to persuade the judge that the conduct at issue is far less severe than most, highlighting Mann Act defendants who profited from commercial sex operations, exploited minors or transported vulnerable victims such as undocumented immigrants.

“Our view is that his case is on the lowest possible end,” Ms. Shapiro said.

Prosecutors have framed the defense’s efforts as minimizing a pattern of serious conduct laid bare at trial. They have reminded the judge in court filings of the vivid testimony from the women about what they endured as a result of the freak-offs. Days without sleep in which they were kept awake by party drugs and had sex with multiple male escorts; freak-offs despite urinary tract infections, injuries or a bout of vomiting; fears that Mr. Combs would follow through on threats to release video of the encounters.

Mr. Combs, the prosecution wrote, “masterminded every aspect of freak-offs.”

The gap between the prosecution and defense views on the seriousness of the Mann charges mirrors the yawning breach in how the two sides view the appropriate consequences for Mr. Combs’s conviction.

Mann Act violations carry a maximum sentence of 10 years each. Prosecutors have indicated that they believe the music mogul should go to prison for more than four years, though they’ve signaled that their formal sentencing recommendation, due next week, could be significantly higher.

The defense, on the other hand, has asked Judge Subramanian in advance of Mr. Combs’s sentencing to go so far as to vacate the jury’s verdict and set their client free.