It was a debate that should never have happened.

Donald Trump will take the stage in Philadelphia on Tuesday night to face an opponent he didn’t expect when he agreed to meet in May. It will be an opponent he has never met and one he finds hard to define: Kamala Harris, the U.S. vice president. Her rise as the Democratic presidential candidate has changed the direction and nature of the presidential election.

The Republican candidate had expected to have an appointment in the ‘City of Brotherly Love’ for a second meeting with Joe Biden, the US president with whom he has had a heated debate history since the 2020 election.

Instead, the unprecedented impact of the June debate between the two in Atlanta — in which Biden’s halting and inconsistent performance led him to withdraw his candidacy after mounting pressure from his own Democratic Party — has left Trump facing an opponent against whom he has yet to establish a solid line of attack.

Harris, for her part, enters the event prepared by aides who have emulated Trump’s often harsh and insulting debate technique — particularly toward women — and buoyed by her experience from a previous career as a prosecutor. She’s also energized by the fact that she’s running against an opponent recently convicted of 34 felony counts.

The two are facing off in a race that multiple polls show is neck and neck, both nationally and in key swing states. In Pennsylvania in particular, the site of Tuesday’s debate, more electoral votes are up for grabs than in any other swing state.

Tuesday’s event, hosted by ABC, will follow the same rules that governed the Trump-Biden debate, with each candidate’s microphone muted when it is their opponent’s turn to speak. Harris’ campaign has advocated keeping the microphones on throughout, hoping to encourage the former president to make the unruly and unwelcome interruptions that have marred his past appearances.

While Trump was willing to agree, his entourage — determined to keep him focused and on message — pushed to keep the original rules.

But according to Steven Fein, a presidential debate specialist and psychology professor at Williams College in Massachusetts, it is precisely Trump’s difficulty accepting Biden’s exit from the race that could determine the contours of the debate.

“I think the most interesting and potentially explosive element of it is the fact that he was clearly very upset that Biden withdrew and was replaced by Harris,” said Fein, who suggested the debate had greater potential for mind games and drama than anything he had previously studied.

“It’s going to be a huge task for him to control his urges. When he’s lured… by a woman, he’s usually very mean. And a colored woman is just like the nightmare scenario.

“There’s going to have to be some give and take in a way that there wasn’t in the first debate, where he didn’t have to say much and just let Biden do his thing. So the potential for all kinds of drama is great.”

The former president is preparing for a debate with Tulsi Gabbard, a former Democratic congresswoman who now supports Trump and ran for the party’s presidential nomination in 2020. He memorably clashed with Harris during the primaries.

Jason Miller, a Trump adviser, said during a pre-debate call with reporters Monday that Harris would have difficulty preparing for Trump.

“The fact that Trump asks extemporaneous questions every day (means) you can’t prepare for him,” he said, comparing it to training to prepare for a fight with Muhammad Ali. “You don’t know what his style is going to be. He has a great mix of humor and charm, and also hard facts.”

With Hugo Lowell