For six weeks, Vice President Harris’s campaign has been running circles around President Trump’s efforts. Democrats have raised tons of money, energized their base, and protected Ms. Harris from unscripted interviews and political risks. All the while, Trump’s campaigning has been lackluster.

True, Trump has been through a lot over this past year — indictments, trials, contested primaries, and an attempted assassination; the strain is showing. Yet the time has come for him to make the sale.

Despite Trump’s lack of focus and Ms. Harris’s early momentum, polls continue to show a close race. This is why Tuesday’s presidential debate is so important. It could make or break either campaign.

As we’ve learned over the years, debates are more often lost than won. The Biden-Trump showdown in June proved it.

For Trump, the debate is an opportunity to recharge, a chance to unveil a strong, disciplined message. For Ms. Harris, it’s an opportunity to broaden her base and reassure skeptical voters.

Pundits tell us, incorrectly, that voters have already made up their minds, but the truth is otherwise. Recent polling finds that a sizable part of the electorate — 18 percent — could still swing either way.

Right out of the box, each candidate should lay the predicate for the entire night. Starting aggressively is the best way to take the offense, regardless of questions asked or time limits.

Debate messaging should reflect the campaign’s central theme — assuming a campaign has one. Trump’s central theme has yet to be honed, so the debate is an opportunity for him to do so.

Ms. Harris’s messaging has been working well, but it’s hollow, mostly a collection of slogans. This debate is a chance for her to tie her themes together.

The first words out of Trump’s mouth should frame the election as a choice between his record and the Biden-Harris record. Polls tell us that most voters prefer the Trump record, especially on the economy, immigration, and national security. This could be Trump’s version of President Reagan’s famous debate line, “Are you better off than you were four years ago?”

Trump also needs to make the case that Ms. Harris is too far left by citing specific policies she’s supported. He should end a line of rapid-fire attacks with a pithy summation, something like: “Biden went along with the progressive left, but Harris will be their champion.” 

Ms. Harris’s first words should focus on the future, reinforcing her message that “we’re not going back” to the Trump era. From the start, she needs to inoculate herself against likely attacks; that will make it easier to respond when they arise.

She will have to be ready to defend her own views — from Gaza to Ukraine, taxes to fracking, grocery prices to deficit spending. Ms. Harris needs to show a depth of knowledge she rarely displays.

At times, the most important thing Trump should do is shut up. Time limits and microphone mute buttons may help him do that. He needs to avoid distractions, such as relitigating the 2020 election.

There is no reason for him to bring up his criminal cases. These are potential traps; Ms. Harris will have ready-made retorts for all of them.

Unpredictability is Trump’s strength. Ms. Harris expects him to be rambling and disruptive. Only if, instead, he’s rational and concise, and his attacks are coherent and focused — not his usual word salad of overstatements and distractions — he could force Ms. Harris off her game.

Ms. Harris should employ two techniques that could throw Trump off stride. One is humor. Neither Senator Clinton nor President Biden effectively used wit against Trump in previous debates, and it was a missed opportunity.

The second is for Ms. Harris to make her points in the form of questions. As a lawyer and senator, she knows how to do this. Even though debate rules don’t allow cross-questioning, nothing prohibits candidates from posing questions to the audience in the form of statements.

After the endless ads and unremitting smears that often cancel out one another, debates serve as tiebreakers for undecided voters. This may be the only presidential debate left in this election. A bad night for either candidate could be fatal.

Tune in on Tuesday and decide for yourself.

This article was originally published by RealClearPolitics and made available via RealClearWire.