To cage-fight Trump, or talk to 100 million people?

Debates don't often decide the outcome of a presidential campaign — but Tuesday's scheduled collision between President Donald Trump and Joe Biden could be an exception to the rule if Biden comes out a winner.

It has happened before.

In 1980, Democratic President Jimmy Carter and Republican challenger Ronald Reagan were closely matched in the polls before they met for their only debate.

Carter had presided over a disastrous four years, including an economic recession. But undecided voters were worried that Reagan, at 69 the oldest presidential candidate in history, might not be up to the job.

At the debate, Reagan was at the top of his game — and deftly parried Carter's attacks with a line he delivered more in sorrow than in anger: "There you go again." A week later, the challenger won in a landslide.

On Tuesday, Biden will be aiming for that kind of outcome.

Like Reagan in 1980, he's challenging an incumbent who's broadly unpopular. Unlike Reagan, he has a clear advantage in most national polls — but he still needs undecided voters to shift his way in the swing states that will decide the election.

"Biden can't put two sentences together," the president claimed in July. "I don't know if he's all there," he said last week.

If Biden can rise above that very low standard, Trump's decision to lower expectations for him could turn out to be the dumbest move of the campaign.

So Step 1 for Biden is clear. If he performs as cogently as he did in the final primary debates in March or during his appearance at a CNN town hall Sept. 17, he wins.

Trump's goal is to knock Biden off stride — to rattle him with wild attacks, distract him by distorting his positions or make him angry by attacking his son Hunter's malodorous business deals in Ukraine and China.

Biden has a temper, and when it gets the better of him — as it did last year in Iowa, when he called a voter "a damn liar" — he doesn't look presidential.

For more tips on strategy, I consulted two political professionals who have prepared candidates to debate Trump before: Philippe Reines, who advised Hillary Clinton in her 2016 presidential campaign, and Alex Conant, who worked for Sen. Marco Rubio, R-Fla., in that year's GOP primaries.

Reines — who in Clinton's prep sessions played the role of Trump, in an ill-fitting blue suit and too-long red tie — told me Biden needs to find a measured way to parry the president's attacks.

"You can't out-Trump Trump," he warned. "But you can't stand there and be a punching bag, either."

"Biden should begin by addressing Trump directly," he advised. "He should say, 'Before we start, I gotta say something: We're all on to you. A hundred million people are watching tonight, and they already know how you operate. When you say something is fake, we know that means it's real. When you say something is a lie, that means it's probably the truth. If you stand up here tonight and make things up about me, it's not going to work. We all see right through it.' "

Beyond that, Reines said, Biden needs to focus on delivering his core message to undecided voters. "He can spend his half of the debate fighting with Trump or talking to a hundred million people," Reines said. "Which is it going to be?"

"This feels like 1980, when Carter debated Reagan," he said. "People have made up their minds about the incumbent, but they need to be convinced to vote for the challenger. A lot of them have questions about Biden's age and whether he's up to the job. That's his challenge — and his opportunity.

"Trump will do everything he can to make that difficult by throwing the kitchen sink at Biden," he added. "You can't ignore him, but counterpunching doesn't work, either; Trump loves it. So Biden needs to dispense with Trump's attacks without playing Trump's game. We never really figured out how to do it."

Here's my advice to the two candidates.

For Biden: Don't try to land a knockout blow; Trump has done a fine job defeating himself without your help. Your goals are to establish your fitness to be president and to remind voters how you got this far — by being a reassuring, empathetic moderate who can organize a coherent response to the coronavirus and Make America Normal Again.

For Trump: If you want to expand your support beyond the 43% you have, here's another chance to be presidential. You have a solid conservative message on the economy, but your penchant for creating chaos and division gets in your way. Above all, stop refusing to say you'll accept the results of the election if Biden wins. It's OK to say you want the election to be honest and fair — but not to say that if you lose, the election must have been rigged.

Doyle McManus is a columnist for the Los Angeles Times. Readers may send him e-mail at doyle.mcmanus@latimes.com.