WASHINGTON — Democrats’ giddy optimism about the prospects of Vice President Kamala Harris beating former President Donald Trump — well, that bubble will burst if the vice president does not do well in the debate Tuesday night.

Doing well is not easily defined. As the expression goes, we’ll know it when we see it.

It could be a lack of a mistake. It could be a one-liner that speaks volumes. It could be silence in the wake of insults. Or an effective takedown. It could be who relates best — a sort of gut-check appeal — to the sliver of undecided voters in the swing states who will determine who the next president will be.

The winner is unlikely to be the one who best describes tariffs or capital gains tax policies. Doing well is explaining stands on complicated policies either simply or vague enough to do no harm to the No. 1 goal of Harris and Trump in the debate.

And, from the perspective of Harris and Trump, the goal of the debate is to use it to help win enough votes in the battlegrounds to get to the 270 electoral votes needed to be sworn-in as president next January.

The 90-minute showdown (with two commercial breaks) at the National Constitution Center in Philadelphia, to start at 8 p.m. Chicago time, is the only one Trump and Harris have booked so far. ABC News is hosting the debate and is allowing other networks to use its feed.

Observations:

Reality check, national poll: The truncated Harris campaign — born July 21, within hours of Biden dropping out — comes as a new national poll of likely voters by the New York Times and Siena College shows a statistical tie. The poll revealed that voters want to know more about Harris. She did a terrific job at the Democratic National Convention in Chicago last month introducing herself. At the United Center, Harris had lots of time, bio videos and a script. Plus, a loving husband, Doug Emhoff, telling the story of their romance.

Let’s see how in the debate, where there will be time constraints, Harris gets people to know her better — and what she picks to highlight.

People are familiar with the Trump story. His poll numbers and base have held despite his lies about his wealth and supposed self-made success. Trump constantly brags about his greatness. If he takes that route at the debate, he will provide Harris with an opening. Last February, a New York judge ordered Trump to pay $355 million in penalties in a civil fraud case against Trump and his real estate company.

One of the things Harris has — that Trump does not — is experience living in the real world without wealth. She cooks. People relate to food. When she took a break from debate prep on Saturday, she stopped by a spice shop in Pittsburgh. And though the visit was contrived, the five seasonings she bought were not: According to the pool report, that’s Creamy Peppercorn Dressing Base, Fox Point Seasoning, Trinidad Lemon-Garlic Marinade, Turkish Seasoning, and Tuscan Sunset Salt Free Italian Seasoning.

Reality check, battleground state polls: According to a new CBS/YouGov poll Harris has a tiny lead in Michigan and Wisconsin and is tied in Pennsylvania. Neither candidate has room for error.

Taking the bait: Presidential debates are performances. In my analysis, Trump seems easier to trigger into saying something ridiculous — and giving himself a self-inflicted wound — than Harris.

How will Trump pronounce “Kamala”: Trump has made a point of deliberately mispronouncing the vice president’s name. Or calling her Komrade or Comrade Kamala — he spells it both ways in his posts. Trump mangles Kamala as a way of trying to minimize the vice president and frame her as an “other.” In an inspired bit at the convention, Harris’ two grandnieces along with actor Kerry Washington taught the crowd in the United Center how to pronounce Kamala as “Comma-La,” at the same time sticking it to Trump with grace and wit.