With smirks and head shakes, Harris uses the split-screen format to provoke Trump

It began just as they appeared before the audience.

Vice President Kamala Harris walked across the stage into former President Donald Trump's space, reached out her hand and introduced herself.

Trump, visibly taken aback, shook her hand.

The assertive move by Harris was one that ended up setting the tone for the next 90 minutes, where Harris made her case to become the nation's next president as much through body language as she did through words.

And in that period, she repeatedly got under Trump's skin, sending him into angry screeds where he struggled to stay on topic and, at times, careened into confounding anecdotes.

“They’re eating the dogs, the people that came in, they’re eating the cats. They’re eating, they’re eating the pets of the people that live there,” Trump said, referencing a baseless claim about Haitian immigrants in an answer about immigration. “This is what’s happening in our country, and it’s a shame.”

Trump spent much of the night leaning forward, hands on the lectern, brooding. He would not look at Harris, even when making an emphatic point about her, instead pointing his finger over at his Democratic opponent without turning his head.

It was one of the most tangible signs that Trump grew unnerved in their first debate, which started with Trump talking on point about one of his key campaign themes: the economy.

Harris at times tilted or shook her head in disbelief, smiled incredulously and at one point looked right at Trump, putting her hand to her chin in an exaggerated gesture as though to convey she was listening to a tall tale. Her expression turned into an instant meme on social media.

As Trump boasted about how he won the 2020 election and had garnered more votes than any other candidate, Harris responded that he had lost — and been fired by 81 million people.

“Clearly, he’s having a very difficult time processing that,” she said.

Overall, Harris' performance was a dramatic shift from what Democrats saw from President Joe Biden in the June debate, which was roundly panned as a disaster for him and set into motion a party revolt that saw Biden drop out of the 2024 election. Then, Biden looked off-camera repeatedly, appearing lost at times and sometimes struggling to complete a sentence.

It appeared Harris learned from that debate: that the cameras are always on and body language was a critical way to communicate with the audience. Harris' decision to initiate the handshake, moving toward Trump, came after she, like other Democrats, had watched Trump use his physical heft in a 2016 debate against Hillary Clinton to project dominance.

On several other occasions, Harris made a point to look directly at the camera while delivering her answers. It was her attempt to try to emphasize the message that Trump is in for it for himself and she sought to turn the page.

But Trump also began delivering circuitous answers — and growing visibly agitated. That included when Harris chided Trump, saying world leaders were laughing at him and America, for ever taking him seriously. Trump then announced he had the backing of Hungarian strongman Victor Orban.

Even some Republicans, at the debate's conclusion, lamented Trump's performance. One Republican donor called him “wild, uncontrolled” and declared that Harris was adept at “pushing his buttons."

“Trump is so angry he can’t clearly get his message across," a Trump fundraiser said. "She’s cool, calm and able to provoke him. I was stressing hearing it. On the other hand, everyone watching are stressed and angry. Maybe they very well identify with Trump’s anger.”

A Republican operative gave a more nuanced answer, saying Harris had not given clear answers herself.

“He is taking her bait — which is a missed opportunity but she’s not giving answers to lower prices and securing the border,” the person said.

Another Republican though, said Harris' expressions may have turned off some voters.

“Trump was strong and measured for the first 30 minutes. He let the moderators fact checking get under his skin for the next 30 minutes," the person said. "And then came back strong after the commercial break. Kamala’s facial reactions in the split screen have been off-putting.”

Trump also complained about the size of Harris' rallies, accusing her without evidence of paying people to attend.

At one point, Trump did glance over when Harris attempted to speak into her microphone that was muted.

"I'm talking now," Trump said, in a reference to what Harris said to then-Vice President Mike Pence in the 2020 election. "Does that sound familiar?"

One Democrat who had been critical of Biden's debate performance had a drastically different view of Harris'.

"Kamala Harris may be delivering the best performance by a Democratic presidential candidate since Bill Clinton. Never seen Trump backpedal like this," said Democratic strategist Pete Giangreco. "That was a flat out a-- whipping."

This article was originally published on NBCNews.com