The first debate is typically the most dramatic occasion of every general election presidential campaign. Two (or three) rivals who have been contending with each other from a distance finally have to confront each other face to face, with the nation watching raptly and the election hanging in the balance.

It's great theater, particularly this year when Donald Trump and Joe Biden square off in what could be an epic brawl. The 90-minute forum, to be held Tuesday evening in Cleveland, will undoubtedly produce a large audience. The initial confrontation between Trump and Hillary Clinton in 2016 drew 84 million viewers, more than any previous debate. This one will dominate media coverage for days.

But here's the thing: It's essentially irrelevant to the outcome. Whatever voters learn from the showdown is not likely to affect how any significant number of them vote. The perceived importance of this debate is a classic case of what is visible being confused with what is decisive.

Recent elections confirm all this. Clinton, according to post-debate polling, won all three debates by a hefty margin, but not the election. In 2004, John Kerry got the best of George W. Bush in the first and third debates, for all the good it did him. In 1984, Walter Mondale outshone Ronald Reagan in their initial faceoff, just a month before Reagan's landslide victory.

There are examples on the other side, such as Barack Obama losing the first debate with Mitt Romney but winning the next two, on his way to reelection. Bill Clinton was judged to prevail in both of his 1996 debates with Bob Dole, and he, too, was reelected. But there's no reason to think the debates had anything to do with the final result either year.

These events can certainly make a difference in primary contests, when voters are getting to know the candidates and still have relatively open minds. Trump owed his 2016 nomination partly to his combative performance in GOP debates — and to his opponents' reluctance to take him on.

This time, Michael Bloomberg looked as if he might be a formidable contender until his first debate appearance in February when he ran into a buzz saw. The headline in The Guardian read, "How Elizabeth Warren destroyed Mike Bloomberg's campaign in 60 seconds."

But most voters were just getting to know the former New York City mayor. That makes all the difference. A debate can have a big effect on how people form their opinions of candidates, but it rarely makes much difference after those opinions have already formed. Trump and Biden are as familiar to the average person as Coke and Pepsi.

The exaggerated importance of presidential debates is partly the result of the famous 1960 match between John Kennedy and Richard Nixon. JFK came across as cool and confident while his opponent looked sweaty and unshaven. But the political environment of that era was radically different.

Television coverage of politics was far skimpier. Americans had not watched countless hours of Kennedy before that first debate; he was still a comparatively unknown quantity. The debate was his chance to make a good first impression on a lot of voters, and he made use of it.

Nor did Kennedy and Nixon have to contend with an avalanche of post-debate analysis by dozens of cable news reporters and commentators. In 1960, Americans had to decide the winner on their own. Today, they'll get an excess of guidance, most of which will serve to confirm voters in what they already believe.

As presidential historian Richard Norton Smith told me, the parties were far less polarized then. There were conservative Democrats and liberal Republicans, who might be lured to the other party's nominee with the right message. In addition, he says, "there were a significant number of undecided, or emotionally uncommitted, voters."

Not anymore. Two months before the 2000 election, 21 percent of likely voters said they might change their mind about their preferred candidate. Last month, however, that figure was just five percent. In this debate, the candidates won't have the opportunity to appeal to a lot of Americans who are wrestling with their decision. The vast majority of minds are already closed, and no burst of eloquence from either candidate is going to unlock them.

Tuesday's debate may offer a rollicking good show. But it's fool's gold. You can admire the glitter, but you can't put it in the bank.

Steve Chapman blogs at http://www.chicagotribune.com/news/opinion/chapman. Follow him on Twitter @SteveChapman13 or at https://www.facebook.com/stevechapman13. To find out more about Steve Chapman and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.