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The world knows what kind of debater former President Donald Trump is: He’s sloppy with facts, quick to insult, and extremely self-assured.

But what about Vice President Kamala Harris?

Though her 2020 presidential campaign barely got off the ground — she ended her campaign in December 2019, before the first primary votes were cast — Harris has made her mark in one important way.

During the June 2019 primary debate, before she was his running mate or anywhere near the White House, Harris launched a fierce attack on Joe Biden.

Policing and race played a major role in the 2020 Democratic primaries.

“I don’t believe you’re a racist,” Harris told Biden, staring at him from the debate stage, looking straight ahead or down at his podium.

But it was painful, she said, that Biden would praise men like the late Senators Strom Thurmond of South Carolina and John Stennis of Mississippi, “who built their reputations and careers on the segregation of races in this country.”

She noted that Biden had worked with these men during his long career in the Senate on legislation against mandatory busing in local school districts.

“There was a little girl in California who was in the second grade that integrated her public schools, and she was bused to school every day,” Harris said. “And that little girl was me.”

It was a powerful moment, a testament to what Harris can do on the debate stage: clearly rehearsed, effectively deployed, uncompromising and outspoken in the face of her opponent, who would later name her as his running mate.

Harris has undoubtedly spent her debate prep time working on material to use against her rival this year. She and Trump will meet and debate for the first time Tuesday night in the key state of Pennsylvania. ABC News will broadcast the event.

Related: CNN’s political team compares Harris’ preparations during a debate camp-style meeting in a Pittsburgh hotel to Trump’s more informal approach.

Democrats have tried to paint this campaign as one between a former prosecutor in Harris and a convicted felon in Trump. Harris will have to live up to her reputation as a tough prosecutor on Tuesday, when she gets a rare chance to vent Democrats’ years of anger toward Trump to his face.

Unlike Biden, Harris doesn’t have to go back to the 1970s to come up with lines of attack. She can look to his criminal conviction in New York, his accountability in a sexual assault and defamation case, his nationalist policies, his baseless claims of voter fraud — for which there is no evidence — or his outrageous promise to jail election officials.

While Harris will have plenty to say about Trump, she won’t benefit from his interruptions. Candidates’ microphones are muted when it’s not their turn to speak, so Trump won’t be able to interrupt Harris with insults as he did with Hillary Clinton, telling her, “You’re the puppet!” or “You should be in jail” during the 2016 debates.

It also means that Harris’ memorable moment in the 2020 vice presidential debate – when she said, “Mr. Vice President, I’m speaking” in response to Mike Pence interrupting her – will not happen.

Not all of Harris’ planned takedowns have worked as well as the busing attack on Biden. In another 2019 debate, Harris took aim at Sen. Elizabeth Warren over Warren’s plan to break up tech companies.

Harris attempted to isolate that larger issue into something more palatable by expressing disappointment that Warren wouldn’t call on Twitter (it was a very different company in 2019!) to suspend Trump’s account. Twitter would later suspend Trump’s account following the January 6, 2021, Capitol riot, but the company was subsequently bought by Elon Musk and rebranded as X — and Trump is now active on the platform again.

Harris’ point seemed small, but Warren was able to make it into a much bigger point.

“Look, I don’t just want to drive Donald Trump off Twitter, I want to drive him out of the White House,” Warren said.

That exchange could be a warning to Harris not to get bogged down in details, because Trump won’t. And if history is to be believed, he’ll happily make up facts to support his positions.

Which brings us to a third telling Harris debate exchange from 2019. She was criticized by former Rep. Tulsi Gabbard over her record as a prosecutor, which in 2019 was more to the right of the Democratic Party. Gabbard argued that Harris was too tough on marijuana offenders and had other criticisms of her time as prosecutor.

“I’m proud of that work,” Harris shot back, arguing that as attorney general she had worked to make California better, that she opposed the death penalty and that she didn’t just “make nice speeches or sit in a legislative body.”

The irony is that Gabbard, now a former Democrat, has voiced support for Trump, who likes to say he would implement the death penalty for drug offenders.

Gabbard is said to have helped Trump with his own debate preparation, and Trump will want to portray Harris as someone to the left of the American mainstream — someone who changed her positions in 2019 out of political opportunism and who has now changed them again to run for president.

Ultimately, the importance of this debate may lie in how the few undecided or volatile voters view Harris, as perceptions of Trump seem set in stone.

In a New York Times/Siena College poll released Sunday, the race is effectively tied, within the margin of error. Few likely voters — less than 10% — said they feel they need to know more about Trump in the poll. More than a quarter, 28%, said they need to know more about Harris, perhaps suggesting she has more room to grow, or fall, after getting a chance to take on Trump.