A spectators' section of Atlantic City, N.J.'s Convention Hall jammed with senior citizens before the third session of the Democratic National Convention was due to start on Aug. 26, 1964. (Anonymous/AP)

In the film, based on a Gore Vidal play that’s being revived on Broadway, the front-runner for the nomination is one William Russell, an intellectual who pokes fun at the cliche’ that he’s “never met a payroll” and stumps the unread reporters, including a few ladies in homely hats near the back of the pack, by quoting Bertrand Russell. (“How do you spell that?” they wonder.)

Played by Henry Fonda (and based in part on Adlai Stevenson), Russell has a history of indiscretions but integrity to spare — unlike his Nixon-meets-McCarthy rival, Joe Cantwell, played by Cliff Robertson, who is faithful to his clingy wife but would stop at nothing to win.

Robertson threatens Fonda that unless he drops out, he will distribute packets detailing Fonda’s long-ago “nervous breakdown” to every delegate in the hall. But Fonda’s aides have something on Robertson, too: Proof that he had an affair with a man during World War II. Will Fonda resort to such low tactics or hold onto his principles?

This same “will he win the race but lose his soul?” bone has been well-gnawed in works about the political world — “Advise and Consent,” “Ides of March,” “Primary Colors,” “Wag the Dog,“The West Wing.” But in real campaigns, how often does a race turn on such a dramatic moral decision?

If you guessed not very darn often, you’re right. “There certainly aren’t any Henry Fondas around anymore’’ who would refuse to drop the dirt, said Mark McKinnon, a political adviser who has worked for such Republicans as George W. Bush and John McCain and Democrats, such as the late Texas governor Ann Richards.

“The game has changed,’’ he said in an e-mail. These days, “everyone knows everything about everybody. There are no secrets. And so, I think, unlike in ‘The Best Man,’ candidates today just assume the worst will come out and aren’t particularly sensitive to dropping bad news on their opponents.”

Meaning everybody does it and it’s no big whoop?

“I think everyone does it . . . And I think everyone hates it.”

Well, maybe not everyone. Chris Lehane, who has worked for Bill Clinton and Al Gore — and so revels in his reputation as a practitioner of the dark political arts that some reporters sent him a copy of “The Best Man” for a wedding gift — said that another departure from the movies is that presidential candidates are kept as far away from such dirty details as possible.

“No operative worth his or her salt is going to expose his candidate; the candidate needs to be in a position to have not just plausible deniability, but in fact reliable deniability. Now, of course, smart candidates who know what it takes to win in modern politics make sure they find themselves talent that is capable . . . but they are not involving themselves in the special ops.”

So do operatives ever have those “Will I or won’t I?” moments in the candidate’s stead? They do, Lehane said, mull some questions: Who threw the first punch? Will it make a material difference in the race, or is it gratuitous? Does it involve civilians? If so, is there a way to limit the exposure of “non-combatants”? And what’s the risk of the mud splashing back on your own guy?

But Lehane, who has co-written another campaign movie — “Knife Fight,” due out in the fall — said he never spent a lot of time worrying about it: “When I worked on behalf of President Clinton and later Al Gore, my perspective was that I was working on behalf of a candidate who was standing up for people who otherwise wouldn’t have a voice, and against some very powerful forces who were doing everything they could to tear down a guy they couldn’t beat at the ballot box, so . . . I played to win, and looked to hit back harder, and slept very well at night.”

The negative intel more often involves policy than past affairs, though.

And so far in the 2012 presidential campaign, some of the most damaging information has come out of the candidates’ own mouths, including Mitt Romney saying he likes “being able to fire people” and Rick Santorum’s unpresidential observation that John F. Kennedy’s speech on the separation between church and state made him want to “throw up.”

Former GOP contender Herman Cain dropped out of the presidential race following allegations of sexual harassment and an extramarital relationship, yet might have survived those accusations had they not come in tandem with a string of foreign policy flubs — and had his campaign done due diligence on its own candidate and been better prepared.

Sex scandals of the “Best Man” variety are not the ammo they used to be, said Michael Rejebian, who with Alan Huffman wrote “We’re With Nobody,” a new book about opposition research.

(Perhaps an example is Newt Gingrich’s ex-wife Marianne, who seems to have been walking around all those years thinking she could end her former husband’s presidential hopes whenever she wished. Yet when she told reporters just ahead of the South Carolina primary that he’d asked for an open marriage as their union was falling apart, Republicans initially responded by rallying to his side.)

Nor is a history of depression the nuclear bomb to a campaign that it was in “The Best Man’’ — or in 1972, when Jack Anderson’s story that Tom Eagleton had had electroshock treatments forced him off George McGovern’s ticket.

On the congressional level where Rejebian works, always for Democrats, candidates know what dirt they have on their opponents and are in on decisions about what to do with it. But when I asked him to share some examples of situations from the past 18 years in which candidates had balked on ethical grounds, he was flummoxed. “Where they said, ‘I’d rather lose than use this?’ ” Uh-huh. “I can’t think of any.”

Melinda Henneberger is a Post political writer and anchors ‘She the People.’ Follow her on Twitter at @MelindaDC.