In a rare moment of late-night television crossing into the realm of high-stakes political theater, Jimmy Kimmel devoted nearly 10 minutes of his Monday broadcast to a searing, meticulously crafted monologue that appeared to reference an alleged romantic relationship between former President Donald J. Trump and Margo Martin, one of his longest-serving and most trusted aides.

The segment, which drew an estimated 4.2 million viewers, blended sharp satire with pointed insinuations that many in Washington and at Mar-a-Lago interpreted as a deliberate effort to thrust the rumor—long whispered in conservative and media circles—into the mainstream. Kimmel’s delivery was calm, almost professorial, but the content was anything but subtle.

“Donald Trump has spent years telling us he’s a man of loyalty,” Kimmel said, pausing for effect. “And he is—loyal to certain people, at certain times, often late at night, in certain Palm Beach residences.” The audience erupted. He then moved into a series of impressions, mimicking Trump’s familiar cadence while describing “a young, loyal staffer who never leaves his side, who travels with him, who answers his calls at 3 a.m., who knows where the bodies are buried—and maybe a few other things.”

People familiar with the matter, speaking on condition of anonymity to discuss private conversations, said Mr. Trump watched the broadcast live from Mar-a-Lago. Two individuals close to the former president described him as visibly agitated, pacing the room and at one point throwing a glass against a marble fireplace. “He was screaming at the television,” one said. “He kept repeating, ‘This is fake news, this is all fake news.’”

Ms. Martin, 36, has worked for Mr. Trump since 2016, first as a press aide during the presidential campaign and later as a senior communications official in the White House. She has remained a fixture in his orbit ever since—traveling with him on virtually every trip, appearing in countless photographs at his side, and serving as one of the few aides granted consistent access to the former president’s private schedule.

The rumor of a romantic relationship between Mr. Trump and Ms. Martin has circulated for years, first appearing in obscure online forums and later gaining traction in tabloid coverage and among political operatives. Neither Mr. Trump nor Ms. Martin has ever publicly addressed the speculation. Ms. Martin did not respond to requests for comment on Tuesday.

The Kimmel segment has now amplified the rumor to an unprecedented degree. Within hours, clips were shared millions of times across social media platforms, and cable news channels devoted entire segments to analyzing the implications. Some Trump allies dismissed the monologue as “desperate late-night comedy,” while others privately expressed frustration that the show had given new life to a story they had long hoped would remain in the shadows.

For Ms. Martin, the fallout has been immediate and personal. Friends say she has been inundated with messages and calls from reporters and strangers. Paparazzi have been spotted outside her apartment in Palm Beach, and she has reportedly limited her public appearances. One associate described her as “shaken” and “furious” that her name had been dragged into a public spectacle.

The episode comes at a delicate moment for Mr. Trump, who remains a leading figure in national politics as the 2028 presidential cycle begins to take shape. His inner circle has worked for years to project an image of discipline and family unity, particularly after the legal and personal turmoil of his first term and post-presidency years. The suggestion of a secret affair with a longtime aide threatens to reopen old wounds and fuel new speculation about his private life.

Late-night shows have long targeted presidents, but few have matched the precision and apparent research that went into Monday’s segment. Kimmel referenced specific travel schedules, hotel stays, and even seating arrangements on Mr. Trump’s private plane—details that suggested the writers relied on public records or widely circulated reporting.

For now, the story remains in the realm of insinuation and rumor. But in today’s hyper-charged media ecosystem, insinuation often travels faster than fact—and once a name is attached to a rumor on national television, it can be difficult to unring the bell.