When Hillary Clinton takes the podium at Monday night’s first presidential debate, she could be staring down an old romantic rival sitting in the front row.

Gennifer Flowers, who carried on a 12-year affair with Bill Clinton when he was attorney general and governor of Arkansas, might attend the debate at Hofstra University as Trump’s guest, BuzzFeed reported Saturday.

“Ms. Flowers has agreed to join Donald at the debate,” her personal assistant, Judy Stell, wrote in an e-mail to BuzzFeed, according to the Web site.

The development grew from a Twitter broadside Trump launched earlier that day.

“If dopey Mark Cuban of failed Benefactor fame wants to sit in the front row, perhaps I will put Gennifer Flowers right alongside of him!” he tweeted.

It was a shot at Clinton’s decision to seat Cuban, a frequent Trump critic, in the front row as her debate guest, and a signal that the Republican nominee might dredge up past Clinton scandals as ammunition.

Trump originally misspelled Flowers’ first name, then sent a corrected tweet minutes later.

The Trump campaign and Flowers did not immediately respond to requests for comment.

The spectacle of the buxom blond lounge singer shooting daggers at Hillary Clinton — perhaps just a few seats away from her former lover, Bill Clinton, who is expected to attend — would only draw more eyeballs to a debate already expected to attract 100 million viewers.

The 90-minute face-off in Hempstead, LI — the first of three planned presidential debates — begins at 9 p.m. Monday, moderated by NBC’s Lester Holt.

The 1,100-seat venue will be packed with 300 Hofstra students and about 400 invited guests for each campaign.

Coming only six weeks before Election Day, Monday’s contest could be a ­defining moment in the presidential race.

And it has already ­become a pop-culture ­phenomenon.

“It’s a reality-TV show equivalent of ‘Who Shot JR’ from ‘Dallas,’ ” said political consultant Susan Del Percio. “Everyone wants to see what Donald Trump is going to do and how Hillary Clinton will respond. He is setting the tone of the debate.”

But getting ready for their shining moment is no easy task.

“Prepping for a presidential debate from a communications standpoint is one of the hardest things in the world to do,” Democratic consultant Dan Gerstein said. “You have to know your own policies, your opponent’s vulnerabilities, what your message is, and how to play defense.”

Trump’s advisers are worried he’s doing too little to prepare.

The mogul has skipped reading policy briefings, eschewed mock debates, and turned away two GOP operatives who offered to help prep him for free, said three sources close to the campaign.

But Trump sees the lack of prep as a strength, allowing him to bring spontaneity into the debate and making him more “unpredictable,” a source said.

But he has been receiving advice from top advisers, like former Mayor Rudy Giuliani, who feeds him potential questions on his campaign plane flights, a source said.

Trump “thinks all he has to do is get four good sound bites and two big hits on her and the public doesn’t care about anything else,” a source said.

Trump has not employed a Clinton stand-in to practice debating against, a standard practice in presidential-debate prep.

His camp’s biggest worry, sources said, is his being challenged by the former secretary of state on ­foreign affairs.

“Substantively, he is behind the curve in shocking ways,” the consultant said. “This could be a disaster, but then again, he’s proven people wrong time and again.”

Meanwhile, Clinton has been cramming like a college student hellbent on graduating summa cum laude.

The Democratic nominee cleared much of her schedule last week to huddle with top campaign staff, read policy briefings and conduct mock debates with Philippe Reines, her former State Department aide who will portray Trump.

But anticipating Trump’s reactions on the debate stage has proven difficult, Clinton allies say.

“It’s still hard to portray him in that kind of setting,” one former Clinton aide said. “You don’t know what he’s going to do or say. The unpredictability factor looms large.”

Clinton, who trails Trump in several state and national polls, may have more to lose if her performance is underwhelming.

“This is her election to lose, and right now she’s in a danger zone,” said Gerstein. “She needs to avoid getting bogged down in a food fight with Trump.”

If Trump, on the other hand, avoids significant ­errors and refrains from personal attacks, he could emerge victorious, said ­observers.

But the presence of Flowers, and the specter of resurrecting 1980s Bill Clinton scandals, could turn the debate into a nasty slugfest.

The revelation of Flowers’ affair with Bill Clinton — the first of many “bimbo eruptions,” as his inner circle called them — nearly derailed his White House hopes in 1992, one month before the New Hampshire primary.

Rumors of infidelity had haunted Clinton from the moment he announced his candidacy, but when Flowers told a supermarket tabloid that she and the governor had carried on a torrid, decade-plus relationship, he and wife Hillary sat for an awkward “60 Minutes” interview to save the campaign.

Bill Clinton admitted that he had “caused pain” in his marriage, but would not confess to the relationship with Flowers — or admit to having obtained a state job for her.

Instead, he maintained that his private life had no place in the presidential campaign and refused to say whether he had ever committed adultery.

The next day, a furious Flowers called a press conference at New York’s Waldorf-Astoria Hotel to tell 200 reporters that Clinton had been “absolutely lying” by denying their affair.

She played 15 minutes of phone conversations between the two in which Clinton instructed her to lie about having been his lover.

“If they [reporters] ever hit you with it, just say ‘no’ and go on — there’s nothing they can do,” said a voice that she claimed was ­Clinton’s.

“The man on ‘60 Minutes’ is not the man I fell in love with,” Flowers told the press. “I would have liked to think, after a 12-year relationship, he would have had the guts to say: ‘Yes. I had an affair with this woman. But it’s over. And that’s the truth.’ ”

Clinton remained in the race, though his poll numbers took a beating. He was dubbed “The Comeback Kid” when he took second place in New Hampshire and went on to win the Democratic nomination.

Six years later, Flowers was vindicated when lawyers for Paula Jones, who had accused Clinton of sexual harassment, forced him to admit under oath that Flowers had been telling the truth.