So here we are again, facing another debate thought to be crucial to the outcome of the 2024 presidential election. When Donald Trump and Kamala Harris take the stage tonight, September 10, at 9:00 pm ET on ABC under the same conditions as the showdown between Trump and Joe Biden in June, both will be under enormous pressure to dispel widespread perceptions of themselves.

Understanding that his countenance over the decade since he entered politics has been anything but disciplined, Trump has the challenge to come off as presidential and sober as possible. Harris, meanwhile, faces an equally daunting challenge: She needs to prove she can elevate her game beyond platitude-rich word salads, think on her feet, and respond seriously and spontaneously to unscripted questions in a coherent fashion.

For all the arguments between the two campaigns about no audience and muting microphones — both will apply again tonight — the biggest structural challenge for Harris is length. Most observers agreed that she made no major gaffes but won few new supporters and broke no new ground during the 16 minutes she appeared on CNN with VP nominee Tim Walz at her side. Tonight, she must function in the most high-pressure setting possible without her running mate – for five times longer, 90 minutes, under the klieg lights and inevitable withering attacks of the 45th president.

She won’t be able to show she is tough by sassily scolding Trump like she did repeatedly Mike Pence when he interrupted her during their vice-presidential debate in 2020 — because with Trump’s microphone muted while she is speaking, he won’t be able to break in. It’s no wonder that, after Biden fought to have the off-camera mics silenced during his debate with Trump, Harris unsuccessfully fought to have both mics on all the time. It would have made it easier for her to execute her basic strategy, just like Biden’s, of getting under Trump’s skin. But now she won’t be able to theatrically reprimand him to prove how tough she is.

Harris was a prosecutor for much of her career, so at least theoretically, her toughness should not be in question. But Trump may attempt to liken her to nakedly partisan progressive prosecutors like Alvin Bragg and Letitia James who sought to bankrupt and imprison him. And as the highest-ranking member of the present administration not named Biden, Trump will attempt to define her as the radical candidate of 2020 and the  failed border czar as VP. Thus, she must establish her own identity apart from the man she served who singlehandedly revived her flagging career after her nosedive during the 2020 Democratic primary.

Debate Style and Substance

While she has had surrogates all over the media proclaiming that she no longer supports the most radical policies she championed during her previous presidential campaign — most prominently a ban on fracking, Medicare for All, abolishing ICE, and decriminalizing illegal border crossings — Harris will certainly be called on to explain not only what her new policies are but also what sort of intellectual process led her to these revised conclusions. Again, in a debate setting, unlike at a rally or even an interview, clever sound bites will not carry the day. She must demonstrate substance.

Unlike at her rallies, vacuous slogans like “we’re not going back” and “a new way forward” will not cut it because she has not defined what it is we would be going back to (other than Trump himself) nor what the new way forward actually is. Vague, undefined terms such as the rich paying their “fair share” and building an economy “from the bottom up and the middle out” will not work like they somehow did for Biden in 2020 because the voters were well acquainted with him. They don’t know Harris beyond the bookend perceptions of an ineffective and unpopular vice president that top Democrats once considered replacing on the ticket, and the new, improved, joyful version of Kamala rolled out over the last several weeks.

It would be a hard sell for even the most skilled politician to claim a new way forward when he or she is joined at the hip to a declining president and his unpopular policies. Every poll taken during this campaign has given Trump an outsized advantage on the two issues that consistently rank as most important to the electorate: the economy and immigration.

A Surprisingly Fluid Electorate

Are the dynamics of the race likely to change based on this single and — as of the moment only — debate? The answer is probably no for Trump but yes for Harris. After all, the matchup is between one candidate who rarely allows a thought to go unexpressed and has long been an open book to the entire world, while the other is still a mystery. One is trying to prove he is more the president who brought peace and prosperity than the one whose administration is ending in chaos. The other will try to prove she has substance to match the style that has largely defined her vibe-heavy campaign. It will be a heavy lift for both.

Harris should be in a better position than Biden in appealing to women and voters of color, one presumes. But the idea that black and brown people do not face the same abiding challenges in making ends meet as their white neighbors is patronizing and ultimately false. Unless this election is different from virtually every previous one, and human nature has changed, general election voters cast their ballots based on their self-interest. While more exotic issues — such as abortion or a controversial tax plan or treaty — can animate midterm election voters, presidential races revolve around the economy. The exception would be wartime elections such as 2004 and 1968, when the desire for military victory and peace was uppermost on the minds of voters. While the wars in Ukraine and the Middle East that began on the Biden-Harris watch may not be decisive factors in the election, fear is growing over the specter of World War III. Thus, the VP’s comments on Israel and Hamas must be carefully crafted to achieve the all-but-impossible: pleasing Jews and Arabs alike. She also will likely have to explain her support for a Minnesota group that bailed out violent criminal rioters following the death of George Floyd, some of whom went on to commit heinous crimes.

At the same time, Trump no longer possesses one of the greatest advantages he enjoyed over Biden: age. Though he is 78 years old but famously energetic, the age issue has now flipped in favor of Harris. She is 59, neither too young nor too old, and just like her immutable demographic characteristics as a woman of color, she need do nothing to secure her age advantage. It is up to voters who thought Trump was not too old when he was matched up with Biden to ponder whether his age relative to Harris’ is now a negative.

Polling across the board suggests that this race is close but surprisingly fluid. Anywhere from 3% to 5% of voters remain undecided in a battle between two candidates with high negatives, though Trump’s approval in the latest Gallup survey rose to 46%, higher than during both the 2016 and 2020 campaigns. If turnout is anywhere near the 154 million who voted in 2020, this means as many as 7 million voters have yet to decide. So this is crunch time.

Harris’ honeymoon is either ended or ending, even as big media continues to prop her up, and the debate is now her proving ground. Unlike her convention speech and single subsequent interview, she will have no place to run or hide. But when the debate is over, she will most assuredly not stand where she does now. With millions of Americans placing Harris under a microscope for the first time, she will prove either that her many critics underestimated her or were right all along.