Ever since President Joe Biden ended his candidacy and Vice President Kamala Harris became the Democratic nominee, former president Donald Trump has struggled to adapt to the new opponent. Tuesday’s debate in Philadelphia should show what, if anything, he has learned about running against her.

Presidential debates often don’t matter much. The Trump-Biden debate in June in Atlanta was the rare exception: an event that changed the course of history and the 2024 campaign. No one expects the Philadelphia debate to be as cataclysmic for either candidate. But given the state of the race, there’s little doubt that the stakes are much bigger than usual and that mistakes will be consequential.

Trump is often graded on the curve in debates. He gets credit for his ability to dominate a debate stage. That was particularly the case in his 2016 debates against Republican rivals. His missteps, distortions, lies and boorishness are often written off as if they are to be expected.

His performance against Biden in Atlanta was hardly impressive. He issued a buffet of misstatements and flat-out lies that left fact-checkers exhausted. He escaped harsher critiques because Biden’s performance was so startlingly weak. Biden is now gone, and the spotlight will be on Trump in ways it wasn’t before, with more attention paid to his coherence or lack thereof as a candidate.

A series of tests confront Trump heading into Tuesday’s event, about self-discipline, knowledge, his age and acuity, his overall temperament and how he deals with the issues of race and gender. For the past six weeks, he’s often been failing those tests.

His campaign team has been virtually shouting for him to focus on the issues where he has the advantage and Harris the disadvantage. He obliges but seemingly without commitment. He’d rather talk about grievances than issues, and so he struggles to stay on track, veering from one thought to another, disconnected, thought. His base may love it, but that base isn’t big enough to win the presidency.

A Friday appearance, billed as a news conference, is the latest example. For 49 minutes, the former president ranted, rambled, played the victim and veered into topics that have nothing to do with either the campaign or governing.

Then there is the question of what Trump really knows about the issues. He gave a speech on Thursday to the Economic Club of New York. It was a substantive address about policy, but delivered in a lifeless monotone, with Trump reading from his teleprompter on the left and then his teleprompter on the right in almost robotic fashion.

He got a question about affordable child care from one of the club’s trustees, who wanted to know what specific legislation he might propose to help parents pay for this costly expense.

His response was vague, off-point, rambling and ultimately a flight of fancy about what he would do as president. If he had thoughts about the issue, he never gave a hint that the substance registered with him.

“Child care is child care,” he said. “In this country, you have to have it.” Okay. He also said that the cost of helping parents is but a fraction of the money that would be generated for the federal government by “taxing foreign nations” (in the form of higher tariffs that he has proposed). He said his policies would produce so much revenue that he looks forward “to having no deficits within a fairly short period of time.”

Debates aren’t graduate seminars about the nitty-gritty of policy, but they do demand of the candidates an ability to speak in more than the vaguest of terms — and grounded in facts. Suggesting no budget deficits in the near future fails the minimum test of credibility.

Beyond being uninformed, Trump has tried his best to muddy his positions on controversial topics. Abortion is the best example, where he’s taken credit for ending the constitutional right to abortion with his Supreme Court appointments and taken a series of positions on what kind of state-based policies he supports.

It’s possible he will be forced to be more definitive on that and other issues. If he keeps tacking to the center, as he’s tried to do on abortion, his base will be unhappy. If he doesn’t, then he could confirm criticism that he and his allies would pursue an extreme agenda in the White House and would be a threat to the future of the country.

Trump’s attacks against Harris have been nasty and personal. He has an instinct for the gutter when dealing with an opponent, and Harris is no exception. What makes this more fraught for him politically are the issues of race and gender that come into play when the opponent is a Black and South Asian female.

During the 2016 debates with Hillary Clinton, Trump hectored her both verbally and physically, at times even lurking close to her in ways designed to intimidate her. Harris will likely try to provoke him. Can he help himself or is his instinct for the most gratuitous and baseless personal attacks too ingrained?

Last for Trump is how the absence of Biden changes both the campaign and how viewers might assess the candidates on Tuesday night. Trump is now the old, Harris the new. He is old both chronologically (he is 78) and he and his persona have been in America’s living rooms for years. The Trump act is well known, and this election will be a test of whether he is wearing out his welcome. Judgments about his debate performance will factor into that.

With Biden on the sidelines, there will be renewed focus on Trump’s acuity. To some strategists, Trump seems not up to the levels of 2016 or for that matter 2020. Is this because he’s just that much older and showing it? Is it because he cannot move on past the 2020 election, an election fairly won by Biden but about which Trump continues to claim otherwise, falsely so?

The combination of grievance and less focus has made him a different candidate — maybe not to his base but what about voters not fully locked into one candidate or the other and still looking for answers?

As one Democratic strategist, who spoke on the condition of anonymity to offer candid views about the debate, put it, “If he gets confused or gets kind of rattled, he has the ability to come off as angry, as disrespectful.”

Trump’s debate preparation is unorthodox. It involves conversations about policy with advisers and then a series of interviews in which he gets more familiar with the give-and-take of answering questions.

Harris too has much to prove. She was unpopular as vice president until she seized the nomination. Herfavorability numbers have risen, but she remains vulnerable to being defined negatively by Trump before she fills out her profile.

Harris’s objectives are clear. She wants to continue to introduce herself to voters who still don’t know all they want to know about her. She might wish to explain changes in her positions since she was a presidential candidate in 2019, something she only started to do in her recent interview with CNN’s Dana Bash.

She has said some positions have changed but her values have not. The debate will be a place to explain what those values are. Is she a Biden center-left Democrat or a California far-left Democrat? The policies she has outlined to date leave open different interpretations.

She also wants to hold Trump to account on his answers, though she cannot afford to be a full-time fact-checkeronstage. Can she both get under his skin with barbs and jabs and still rise above his personal attacks? She can’t let him control the tone and tempo and will seek to put him in his place whenever she can.

Brett O’Donnell, who has coached various Republican candidates for debates, though not Trump, said the former president’s goal must be to tie Harris to the past four years and the policies that are the most unpopular. He said Trump should write on a pad the words “weak,” “failed,” and “dangerously liberal” and incorporate them into his answers.

“He’s got to make this a referendum on the status quo and convince folks that she’s part of the status quo and that she caused the very problems she’s trying to solve,” he said. “If the debate becomes about personality, if it shifts to persona, that’s bad for him … She wants the debate to be about who do you like more and do you want to go back to him.”

Some strategists believe Harris will be under more pressure than Trump, simply because she is newer, less tested and seeking to define herself as both part of the Biden legacy of the past four years and a candidate ready to stake out her own identity. But with the race as close as it is, Trump could feel the heat just as much.